Yes, I know about cryptotalk.org. They failed to make any impact even after spending so much money paying users.
not only that, they doomed themselves from the get-go, the only reason why people flocked to their platform in the first place was because of the incentive of being paid just by posting. then once they started putting requirements and banning certain countries, the "active" users on their platform started to drop.
Personally, I as a forum owner will never allow a competing forum to advertise on my forum though.
that's good, I mean it's your forum and you'll decide what can be advertised or not.
But if you're worried, I wonder what you're worried about that you don't want to have competitors?
one thing I can think of is OP is afraid of losing active users.