Creating a spammers forum

[Carnage]

You could try something different could be tricky to enforce though.

If in your registration terms you add something like 'This is a link building forum, created with the purpose of allowing people to advertise. You agree to pay me $100 for each link posted' (translated into legalese) make it very obvious; right at the top of the box. Maybe even add a custom question to the registration form with I agree to pay etc with a checkbox.