I'm getting swamped by spam users. Can I use rules to automatically delete users that 1) don't have content/comments, 2) have never activated in a few weeks, and 3) have only activated the same day they created their account, but haven't posted content/comments?
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.
Install honeypot module and also create a rules which banned the IP when it fails on honeypot.
Honeypot have a rules integration, just create a rules and action to banned the IP when it fails on honeypot.
On honeypot, create a meaningful field name like "country" or "city"
I combat the spam bot for a week, and finally found this method to block them. My banned list was growing larger and larger in the database and then I analyze the banned IP addresses and create IP ranges banned in .htaccess file.
This way, you have multiple layer of security.