First, are you POSITIVE they are colluding?
If so, you have a huge problem, but there are diplomatic means to handle it.
first this is your game, so you have to sorta bow here is how I'd handle it.
1.) You sit out next time, and deal. Announce as a general policy that collusion has been raised as a concern and that tonight you will be dealing, and viewing any discards that YOU deam suspect. In other words no one may request that you look at so and so's cards. It is at your sole discretion. When you peek keep it discreet, and look at least a few random peoples cards each hand as well. I promise that you will never have to confront the folks because they will get it, and they will not collude that night. Afterwards you play and specifically watch for collusion techniques. I don't know how good they are but many techniques are pretty hard to catch in general.
"softplay collusion"= oh it's just us? I check then...aka husband and wife.
the above solution works, this isn't a cheating team, just some folks softplayin their friends. After a check down peek at the cards, discreetly. People using this technique are friends, not trying to chase their friends out, this is not malicious, and a pleasant reminder that this is poker will take care of it.
"dump if I raise dude!" This is tough to catch, but is a better way to cheat, as it simply puts the best hand up against the field. This is done by folks a bit more malicious than the first example.
"Dude if I raise, you reraise so we can trap a guy", done by retards this is way malicious and requires "collusion mathematics" in other words if they can't beat the game legit, they probably cant collude very well either. again reserving the right to randomly look at players hands will resolve this. The play to look for is Raise, reraise, rereraise, get everyone out then check it down. These folks should be not invited back, they are cheats period and destroy the integrity of the game.
In the last example I will play against thos colluders, fold everything ut powerful hands, and just call the raise war. At the river reraise for value you will cripple both players severely.
Hope that helps
This is if it is a "friendly" game and you dont want to chase anyone away.