Mafia: general discussions.

If someone cheats and breaks the rules of the game, the GM, for the first time, replaces their role with a random one from the reserve roles.

I don't think it's a good idea to randomly swap a player's team with all their knowledge. This can also break the game, just like a modkill. If the role simply degraded, that would be somewhat acceptable.

It seems like the best solution is to eliminate them completely and create sets of 2 mafia teams, with 2-3 people in each.

A mafia team of two people is just a pair, not a team... And if you make it bigger, you also need to make the town bigger, but there simply won't be enough people for that.