I had a similar problem with user registration spam…it took my web host adding captcha to my login to get rid of the problem…they deleted all the “users” who were all spammers…none of the emails I replied to ever replied…they were all fiction…
I contacted my host Ipage who set up a captcha for me…problem solved…no more notifications have occurred since she did that…
I had a similar problem only the user registrations were numerous…I solved the problem with a “capcha” on my login…
