Track and Delete Abandoned Sites?

I’m using Pay To Blog and thinking about offering 3 free days or something like that. I know that will lead to a lot of abandoned sites. Is there an easy way to see what sites are not paying and have been abandoned? I assume I would want to clear our old sites that never paid or stopped paying months ago.

If they stop paying (I’m using recurring billing) I assume they just lose access to modify the site, the site stays as is on the network, correct?