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Unfortunately no, you can't block them. My recommendation to deal with theses trollers is that you keep deleting and banning them and don't rise or take offence to their antics.

Furthermore make sure "Email activation" is on so they'll need to create a new email to keep singing up to the forum,

You can find this ACP --> Users and Groups --> User Options --> New Account Activation

Secondly as an added security bonus, even though it might not do much you can limit the accounts created under similar IP addresses.

ACP --> General --> Security --> IP address accounts creation limit, for each 24 hours --> Set the number to around 2 or 3

Thank you for that second idea. I have not heard about that one. ONly one issue.
They are posting as a GUEST.
We encourage guests to post in our Advertisement board for their own sites, and we put our add on their sites.

We already ahve the site set to 'Admin Approval' for new accounts.

Maybe for the time being you could perhaps take away guest posting rights just for a short time, then maybe your troller will decide to give up once he sees you've taken firm action. Other than that I can't really suggest anything.

I'll discuss this other option with the other admin. She is the founder, I'm only stepping in while she is away at the moment. Thank you for your help. The troll is unpredictable at best, one day he's here, then gone for a month, then back for a few days then back again in a week or two... Rediculous. He says he's going to SHUT DOWN the forum in November... lol

Try not to react to what the troller is doing and saying.  I would not even reply to the troller's emails, private messages, or posts.  Just keeping banning the accounts and removing the troller's posts.

That's all I have been doing.