How do you prevent Free Trial abuse?

Say you have a free trial and user signing over and over. How do you prevent them from doing that? Going a bit deeper, why are they doing that?

How's

• Require credit card
• Ask for a phone number
• Prevent name+1@mail.com
• Track/block IPs
• Tracking cookies
• Stop offering free trial

Why's

• Free trial is enough
• Price is too expensive
• Payment difficulties

Here is what come to mind but do you have other ideas?

If you're struggling with this issue I'd love to talk here or on Twitter