If you have a website where you require a user to register, maybe to collect details, provide a newsletter etc etc then a couple of hints;
1. Deny anyone who registers an email address with @mailinator.com
Mailinator allows someone to make up an email address on the fly in 2 seconds. They then use this as a one off to register your site, and within an hour - the email address is no longer valid. This means that when you send out a newsletter, it will be bounced, however the person who registered will still have an account or login/password
2. Check
www.bugmenot.com and see if your website is listed.
1 person registers on your site, and shares their login details with the world. This will reduce your audience. I have checked a few peeps sites, and have found about 85% to be listed here.
If anyone else has useful hints/tips, please add....