Were Any of Your Sites Affected By the Penguin?

Because you're holding a site to ransom by charging them remove a link that they don't want on your site and that could be affecting their website's rankings.

Either way, you should remove the links anyway. They're only going to include your website/forum in a disavow list that could potentially harm your site, as Google may mark your website and any links on it as toxic. i.e. a site that is allowing others to buy link placements.

 

You are paying the hosting fees. A good explanation would be that you will be forced to close operations in 30 days if people have a problem with it.

 

See second point.

Although, Google will have no problem with you having paid links on your site, so as long as you nofollow the them.