How can I prevent someone with a stolen device from accessing my village?

You write a message to SuperCell and they will ask you some info like below

• When did you start playing on this account?
• Where (which city) did you create the account?
• When did you notice that your account was compromised?
• What device(s) do you use to play the account?

If you sync your game via your Facebook profile instead of your Game Center the game will prompt you once in a while to re-log since Facebook's authentication has a lifespan of a few hours to days while Game Center is account bound.

Things like erasing your device can only be done if you use iCloud's Find My Device function, not via your phone company.