How do you lock a losing samsung mobile?

Answer 1

Call your carrier and ask them to bar your IMEI, this effectively bricks the phone.

If it is an Android phone you can go onto Google Play on your computer and download apps to lock your phone in the US carriers wont Blacklist your IMEI other countries might.

Answer 2

If you lost your phone, you can contact your service provider to have them block the device from being used by a different person.