The easiest way to stop bots from registering on the site is to make a custom field with a text like "Enter the number five in this field" and then on registration check if the field contains "5".

Those spam-bots are programmed to cope with standard anti-spambot-modules (i.e. simple standard captchas). Make something custom and they'll give up.

Alternatively integrate reCAPCHA - they're one of the best.