The best way to go about a lawsuit is not try taking on the gambling company themselves but to go after their largest accessible advertiser.
Convince a judge or jury (depending on legal system) that the company had reasons to know the marketed client was committing fraud and continued to market them, then provide evidence that financial harm was suffered by x, y and z as a result of the defendants advertising/marketing and you got yourself a case. It might not be a clear cut case but its one with a reasonable enough chance of winning.
There is a case like this in process right now against a gambling commission. Early stages and its likely going to be settled before it goes to court. I can't be the one to report this story but won't be surprised if it pops up soon in a G911 or EOG article at some point but perhaps not because I think they both had been an advertiser of the company involved.
The bigger sites are smart to take
AP down because a) Its probably the right thing to do b) they are under public eye by the players. c) they are the more likely targets to face legal action if this becomes a repeat incident or other problems are later revealed.
I also don't think there is anything wrong with telling and affiliate you think they suck because they are still marketing a site. I know for me I get angry seeing sites push companies like SBG Global because they are known to steal from clients and if they gave me $20,000 CPA per client I would still tell them to go to hell. Meanwhile my competitors are making more than me because there willing to sell out and help frauds to make more money. Yeah that gets me a little angry that I am honest and make less.. But at least i don't have to walk around thinking one of my website visitors might gun me down on the street someday and they do.