Call Madstar Mobile Customer Service and ask to suspend your number. When your phone number is suspended you're still billed the monthly active line fee, but if someone has your phone they won't be able to use any services that cost you money.

This is also a good thing to do if you want to keep the phone number that's on the phone and transfer it to another device at some point. You'll have to un-suspend the number briefly to move it to another phone, but until you have that new device you don't have to worry about charges being racked up.

Just remember, most phone location services will not work once the line is suspended because they require a data connection. However, you can suspend and un-suspend your device as needed.