Bot registration

Hi, we've starting to have a big problems about bot registrations.
We've recaptcha to signup for a blog, and an account, but in the last few days we've starting to have a lot of registrations from different emails and nicks ( abram9897160, anastasia7192003, zavier4737957, melvin987077, jayleen5141016, etc).
For now we're deleting them manually, but it would be better something else, every day we waste a lot of time just to delete them.
How can we resolv this noise?