I have people subscribing to my phpBB forum. They make no post but leave links to fowl websites.. Does this happen to anyone else..
how can I Prevent this
I think (but I am not sure) you can configure phpBB so that newly registered users first have to activate their account by clicking the activition link in an email they get send. If you activate this option you will propably get less rubbish.
Furthermore you can look at the IP's of these malicious users and see if there is a pattern. If so, block the ip's of these 'users'.
There just spammers. You can set it up so that they have to click on a link in the email to activate there account or else you can have the admin manualy activate them and the admin activation way is alot more secure I think but it might get a bit annoying after a while.