I had the same problem. I did two things which stopped it completely. This works guys so give it a try.
1. Install the
Stop the Registration Bots modification.
2. Create a hidden profile field during registration that forces people to answer it.
For #2 I created an anti-bot question and used a PCRE regular expression to force the answer to be an exact amount of characters. My question was a math question with a 3-digit answer. This is the PCRE I used:
^[0-9][0-9][0-9]$
I used to get anywhere from a few to 10 new bots registering daily. Once I did the above it stopped
completely.