I used to run a poker collusion department for a well-known UK poker site so I feel that I am qualified to comment on this. I have absolutely no doubt that collusion takes place on Prominence. It'll happen everyday, I've seen it myself.
1) report button would be good but it's not essential, just screenshot the players involved and send them an email/twitter message. That's what I have done in the past. Player feedback is very important so I urge people to report anything dodgy. Describe the hand, why it looked odd, what time it took place etc. Makes it easier for the guys to investigate
2) banning party chat doesn't really help as people have already stated, telephones/instant messenger exists so cheaters will just use that
3) I really doubt the guys at Promince Poker have a dedicated poker collusion team looking at this. Like I say I used to work for a company dealing with real-money and to say we were understaffed is putting it lightly. I would guess maybe one person looks at it occasionally
4) I would suggest said person look at high stakes heads-up cash games and SnGs (there can't be that many >500k buy-in being played) to see if any suspect play is occurring/chips are being passed. Might help you find some players who have been ganging up on others and are then redistributing the chips. This of course doesn't help with lower stakes but it would be the easiest place to start probably
5) Other thing that should be looked at are whether suspected players frequently sit down at/leave the same table together. That's an easy giveaway that something is dodgy
6) Devs should be transparent with their collusion policy to give players faith in the software. What happens to players found guilty etc. Are players refunded who were victims? Are guilty players banned permanently? This needs to be made clear
If any of the devs ever want any help/ideas on how best to tackle collusion then please feel free to drop me a message.