You might try using a custom user field with a required multiselect option on your signup page, with multiple answers, only one of which is acceptable. That’ll probably slow down the bots a little, or at least give you an easily filterable field to help delete bogus signups. (NOTE: After thinking about it, I suspect this won’t work because there’s nothing to require a specific answer. The method in an earlier response might work better.)
I find captcha-type traps ineffective these days, the bots can solve them faster than I can.
There should be a way to delete the new L0 users in rails, but be sure to do a backup first. If they’ve posted, you’ll have to delete those posts first.
Others have said that requiring two-factor IDs for signups pretty much eliminates the spammers. I haven’t had to resort to that yet.