Automatically delete "empy" inactive websites?


Here is my problem:
On my multisite network, a LOT of people are creating a free website, but never come back again.
The problem is I get as a result some memory problems because of this huge number of websites.
I was just wondering (because I'm sure some people here had a similar problem before) if there is a solution to that?
One could be to force people to pay to have a website (actually I'm considering this option), maybe with a trial period. But if I'm correct there is no way to automatically delete a website if someone creates it and never come back (for example to delete it after a month if the person doesn't come back).

Or to disable it if a user doesn't log in for more than a year, for example.

Any suggestion?

Best Regards