Their presence is disguised. They're creating accounts using gmail addresses and fake names. Edited my post to make that clear.

Make a big deal about the shill/fake accounts. Educate people as to the benefits of your offering vs theirs and explain how even with their "free credits" offer, it's a poor alternative. I presume these offers are made in DM's? I'm not aware of any "forum" type software that allows admins to prohibit DM's.