In other words, they out bid you even if they have a very bad hand just to prevent you from getting any points above the line?
I will expand on Benjamin Cosman's answer by noting that:
The behaviour you are observing is bad bridge; and
The (scoring) line in Rubber Bridge is simply that, and a means (other than a time limit) for ending the match. The winner is determined by the total of points obtained both below and above the line.
However, to expand on point 1 above, learn to double the opponents to truly penalize them for their unsocial behaviour, and enhance your defensive skills to get those extra tricks that turn a collected penalty of 100 or 200 into 800 or even 1100. A couple of those should slow down your opponents penchant for stink bidding.
If collecting penalties is still insufficient, offer to play for a little pocket change. A one cent a point stake over 3 or 4 hours can comfortably net $40 to $80 profit over weak players. There are two common forms of scoring in money bridge: rubber and four-deal or Chicago. Tough money players will deliberately play rubber against weak players as it creates additional strategic considerations that those players fail to grasp. Only against players that they respect do they opt for Chicago scoring, as it reduces the luck element.
Finally, I will note that the scoring of doubled not vulnerable under-tricks was changed about 20 years ago. Formerly all doubled undertricks after the first were scored at 200 points each, but long experience revealed that this was inadequate. A change was made that the 4th and subsequent doubled undertricks not vulnerable undertricks now became 300 points each, the same as when vulnerable. Make sure that you are using the current scoring table - and practice your doubling and card-reading skills.
I fear the other answers might be overthinking this. If your opponents outbid you and you believe you can set them several tricks you double them. This is the element in bridge scoring that increases the penalty your opponents pay for going set in their contract, and it serves as a disincentive for overbidding or over-ambitious "sacrifices" against opposing contracts.
If you find that your opponents don't care about the score penalties they incur when you double them and set them several tricks then you should move on to Benjamin's and "Forget"'s answers.
There are three cases:
1: You are playing at a tournament.
First of all, you're probably not. There are many ways of playing bridge with different scoring systems. For example, in "duplicate bridge", you are scored based on how well you do compared to other players who played the exact same hand - even if you get lucky cards and make a small slam, you lose if another partnership bid and played the same hand to a grand slam. As far as I can tell, the kind of bridge you are talking about - "rubber bridge" - is not often used at competitive levels, presumably because it has more luck than many other systems. (I don't have a source for this though; please correct me if I'm wrong!) However, if you do find yourself playing tournament-level rubber bridge, presumably your tournament has rules in place for ending interminable matches, whether they're caused by sketchy opponent tactics or just weird luck.
2: You are playing for fun, with reasonable opponents.
If you are unable to finish a match, the Laws of Rubber Bridge have you covered:
Law 80 – Incomplete Rubber
When, for any reason, a rubber is not finished, the score is computed as follows: If only one game has been completed, the winners of that game are credited with 300 points; if only one side has a partscore or partscores in a game not completed, that side is credited with 100 points; the trick points and premium points of each side are then added, and the side with the greater number of points wins the difference between the two totals.
So just agree that the match has to end now, award a bonus if appropriate, and see who won.
3: You are playing for fun, with unreasonable opponents.
Seriously, why are you doing such a thing? Here's what the Laws have to say (emphasis added):
The Scope of the Laws
The Laws are designed to define correct procedure and to provide an adequate remedy whenever a player accidentally, carelessly or inadvertently disturbs the proper course of the game, or gains an unintentional but nevertheless unfair advantage. An offending player should be ready to graciously accept any penalty set forth in these Laws or any adjustment or decision of an Arbiter. These Laws do not deal with dishonorable practices where ostracism is the ultimate remedy.
If you have a good hand, you should be able to make a bid or collect an offsetting penalty. So if your opponents consistently overbid, just double them (consistently). If you double them and they "get away" with it (e.g. make a game or lose less than the value of your "lost" game) when you do, you are being outplayed. Unless your opponents are very lucky (e.g. all of their finesses and "drops" are successful).
If you and your partner have 25+ high card points and an eight card suit, that's enough to make ten tricks most of the time. If that suit is a major, then four of a major.
Lets say you can bid and make four hearts. To outbid you, your opponents need to bid four spades (or five of a minor). But if you can make four hearts because you have 25+ high card points, your opponents should be able to make only six tricks in four spades (assuming an eight card suit) with their 15 high card points (or less). That would be down four, or 800 points NOT vulnerable. That one defeat should cost about the same as a "rubber" (two games to zero) and more than a two games to one rubber.
While it is not against the rules to do this, if an opponent keeps doing this, you should call the tournament director. They are able to exclude this person or pair from the tournament because bridge players are expected to be courteous.
Edit: to be clearer, when I read the word "never" in the title, I'm inagining an opponent with a grudge against you who stops at nothing to keep you from playing. Having the worst possible score does not matter to them.