Paypal works for me generally... there's a glitch occasionally when I buy something from ebay - for some reason paypal doesn't recognize my address as verified.
Your site may not condone illegal activity, but operates in a gray area... people will go there to trade illegal software and/or links to illegal software/cracks for pay content. Yes, you may close threads that have links to the above, but it'd actually be better if you moved them to a 'junk' forum that only moderators and admins had access to and deleted the accounts of those participating. I'm not telling you how to run your forum... It's up to paypal to decide if they want to operate in that gray area with you and apparently they choose not to.