Well, in terms of online, I think it is hard to do unless you are on a Discord call or something. I'm not much of a poker player, but what is the benefit of having collusion? Most likely, it is to gang up on one player? I mean, that's just really unfair.
Better chances of winning by manipulating the game with a joint strategy. Perhaps if you are thinking of small bet tables it may not be worth the trouble for you, but think of bigger tables. 8 players, $10k each. Even if all 7 people colluded, it is still $10k to be split among them. Easy money, low risk.
I believe it can be easily controled by casinos of they set their matching systems so people from similar IPs cannot end up in the same table. It would be similar to systems which are used by cooperative games on the internet to prevent players from the same IP address from colluding against other participants.
What do you mean with similar IPs? The same city, region, state, country? Such limits are arbitrary and actually do not prevent collusion.
It would also require casinos not to allow their poker players to make use of VPNs to mask their IP address.
A residential proxy can easily defeat this ban, so neither does this work.