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.

They're actually googling people's names, seeing where they work, deducing their work email, and emailing them.

Looks slightly less shady than DMing them I guess.