Severe issues between my forum and another forum

First things first, I hope this belongs here. I wasn't really sure where to put it. If it's the wrong section, I do apologize.

So basically, I am an admin (not the founder) on a forum. There is another forum similar to ours that has been harassing us for months (sending fake abuses, having their members create accounts on our forum to cause trouble, spreading rumors that we hack members, the list goes on).

The fake abuses include quite a bit of swearing and the use of derogatory terms.

Is there anything we can do to put a stop to this? We've been banning accounts and IPs as we notice them, but these troublemakers just come back by using fake IPs and proxies. It seems like nothing we do helps the situation at all, and as you can imagine, we're sick of it and want to put a stop to it now. It's gone on for far too long.

Thanks in advance.

Sorry, but Forumotion would only get involved if the two forums are breaking the Terms of Service.  

I would try not to interact with these members at all, just continue deleting their accounts and banning them. Also you could have it where new accounts have to be activated by an administrator.