As far as I know, there are no official rules.
There are two issues:
- What to do with the units of the leaving player.
- What to do with the victory condition.
Units of leaving players
You can remove the units. But this changes the board (it opens possible great positions to other players.) This is an advantage to the first player with settlements nearby. So I recommend that you leave the units.
The board is optimized for the number of players (for example 5-6). If you play with less players, the board is to easy. You can solve this by increasing the number of needed victory points. You can use these guidelines":
If the number of players stays within the limit of the board (for example play with 4 players on a board for 3-4, and one leaves), no change.
Else, follow these steps:
- Take the surplus player capacity (number of players needed to be within the range of the board) and multiply that with the original number of needed victory points. Call this V1.
- count the number of settlements and cities on the board that belonged to the player that left and multiply that by 1.5. Round this down and call that S1.
- Subtract S1 from V1 and divide the result by the number of players left, round the result down and call this V2. This is the amount of extra victory points that need to be gained in order to win.
This way, you keep the game challenging and fair.
A game with 5 players on a board for 5-6 players, 13 victory points. One of the players has 5 units on the board and leaves the game.
We now have 4 players on a board for 5-6 players. So V1 = 13 x 1 = 13.
The leaving player has 5 units on the board. So S1 = 1.5 * 5 = 7.
And V2 = (V1 - S1) / 4 = (13 - 7) / 4 = 6 / 4 = 1.
So now each player needs to get 14 (13+1) points to win.
Note, the swap can create an instant win if the leaving player has either the largest knights or the longest road.
Note, because you can only win on your turn, it is possible, that you have enough points, but because the player before you stops, you now need more points to win. If you think this is not fair, you can check victory conditions just before the player leaves. This can cause a tie.