How to handle collusion?

[MCS]

I never play in a cardroom, so I was wondering: what is the best way to handle collusion if you see it in a game?

To me, there are two basic types: (1) naive (or I guess sometimes non-naive) softplaying, where friends routinely check it down once they're heads-up, and (2) evil secret cheating, where people are signaling each other or raising to knock others out and then folding before the river.

I don't know the optimal way to handle either. Thanks.