I'm just curious. Is it a case of forum war? (meaning members from one forum goes to another forum and troll/spam?) did something happen between your forum and the other forum?
as some members suggested, simply moderate the "new user" usergoup. You'll find out who's trolling. I had a similar issue when members from another forum tried to spam our site. I have never banned so many people at once..haha

But in my case, the other forum admin was VERY helpful. He helped me get rid of trouble makers.