stolen iphone

Contact the carrier. Only the carrier has the ability to block a device. If you have activated Find My iPhone before the device was stolen, you can lock and/or wipe the device, but that is not a block. Also, if the device has the latest iOS on it, the thief would not be able to activate the device without you removing it from your iCloud account.

Only if it has been configured with Find My iPhone on iCloud.

Okay, that takes care of your data and activation. However, it does not "block" the phone in the manner I believe you are referring to. That can only be done by the carrier. However, since they would not be able to activate the phone, it really doesn't matter.