I've always hated player trade and it is especially bad when players invoke no kind of trading strategy. Sometimes they'll hand cards over to the player who is close to winning! People seem to think solely about the cards they need, but there are many scenarios where the act of trading leaves you in a worse position overall due to helping another player.
Because of this, when playing on catan universe I have implemented a strategy to discourage player trading. It is as follows.
Focus exclusively on hexes with ore, wheat and sheep. Get your 2 cities down and then relentlessly buy cards.
Meanwhile, publicly keep track of the number of player trades initiated by each player and announce you intend to exclusively attack the player who attempts the most trades.
Keep the robber on that player using knight cards and all your 7s. You will quickly get the largest army and all of the help you need to make up for the absence of lumber and brick from the rest of the deck and the robber.
If they don't have cards to trade, they can't trade.
Don't engage in any player trading yourself. You won't need to anyway.
Where possible rob hexes that will prevent the other player from being able to buy development cards.
Don't do this unless you have at least four probability dots for each of the three types.
Expect lots of anger from the compulsive traders, so don't play like this with friends 😀