by Emma Richardson.
Captcha helps with spammers but does not fix the issue.
The bigger issue is that if you allow students to not confirm their emails, you will be inundated with bounced emails from Moodle. As you already know, people happen to put in the wrong emails all the time. Plus Moodle uses email as the primary form of communication so it makes sense to verify them.
However, that is not an answer to your issue, I would look at the plugins to see if there isn't something there that would work as you want.