Why not report the site and have it taken down ? If its affecting your business and created intentionally to do so,you can serve them right.The accusation here is good enough for people who use the forum ,doesn't really impact the traffic outside which directly play at bitvest. Since I play here occasionally ,I have met people who have no idea about the forum.Including a banner on bitvest about this should be effective.
sorry, that is barely possible if that site is using a VPS or even tor-like server hosting, because it is hard to find the owner or destroy the site, even if made it, the site could be returned again.