Spam users and rules
To get rid of inactive users, have a look at the Inactive User module.

User Stats will keep track of the IP's used by the various users.

The Spambot module is quite effective at blocking bots which create the inactive accounts. If you can take the time, it will let you report and delete spam postings to help block future attempts.

However nothing will stop them. One of my sites just came under attack over the weekend. Over 600 new user registrations in less than 12 hours (at times 3 per minute). I checked the IP's and found that I had multiple accounts created from a few IP's (between 15 and 45 users from the same IP). Manually blocking the IP's involved at /admin/config/people/ip-blocking and deleting the accounts was the only way to stop the attack. 

Good luck.