Spam is a real problem when running a blogging app with a free tier. This probably doesn't come as a surprise. 

I do my best to review new blogs but a lot of backlink nonsense slips through the cracks. I think it would be fair to auto-delete free blogs after a period of time (a year?) if:

This would auto-eliminate most of the under-the-radar spam, although you might argue domain reputation damage would be inflicted much sooner so perhaps I should use some clever analysis that allows me to cancel these accounts sooner?

For the rest, I'd give people a heads-up that their account was due to be nuked, but I imagine this would make few people suddenly start blogging again. They're just tyre-kickers. Given that, this plan seems fair enough wouldn't you say?