Where are the poker freerolls?
Gone.
Removing the freerolls is great and all but then how about the "challenge" rolls? Get rid of the 100 Flops Bronze Challenge. Just do it. If you don't want to spew to the colluders, then you'll get rid of it and force them to play against players that have way higher skill than them.
If you don't want to do this then you need to up the security against collusion and catch them. After all, most of your playerbase is in micro stakes.