Auto delete sites if not upgraded to PRO in X days


I've recently noticed a significant increase in bogus sign ups in one of my multisites.

Though I've imposed a "compulsory" upgrade to PRO in order to continue using the registered sites, but the recent increase in "spam" registrations with seemingly no intention to upgrade to PRO is a concern - that those sign ups are sort of like going for keyword site names like /business, /internetmarketing, /health etc. These prevent genuine signups with intention to convert to PRO the chance of getting such keywords.

Wonder what is the recommended remedy for this ... auto delete both sites and users if they are not upgraded to PRO in 7 days?

Best Regards, William