There are many ways to help prevent spam from signing up using Google captcha and other methods but one good and easy method adding a checkbox to the sign up as bots can’t tick these, yet.
Go to your Admin Panel > Settings > Profile Questions
Create a new heading I.E. Spam Check
Now create profile a question and name it “Please put a check in the box” (or anything that you prefer). Ensure you have selected its a required Question.
Automated spammers can’t check the box on the sign-up page where the question is so they will not be able to complete the signup.
That’s all and your site should see a significant drop in spam signups