i get this a lot on my site too all the anti spam features dont stop it because most of it is someone scrolling through oxwall users websites and manually signing up the only way to stop it is to set mandatory user approve so any new users that join will have to be verified by admin before accessing the signed in version of the site
For some small communities this would be an option but for larger websites not (imagine that Facebook would verify each user account manual...)
So a solution could be: a good BAN/BLOCK function or (and maybe better) a required Two factor Authentication via SMS (which will def. will cost money..)
I see that someone is busy to develop such plugin, but it still needs a possibility to block phone numbers so user can't registers once their number is on a black list.. (Maybe something can arranged via the SMS service provider)
EDIT:
here is an already working SMS verification plugin, but still I don't know if blocking numbers is possible..