To knock out an enemy's radio easily, when you hold them up shoot it (on the belt to the right of their back). Also shoot an enemy's radio when they are knocked out to prevent them from calling for help if they wake up.

If you shoot at an enemy's radio when he is calling for backup, it will be destroyed and you will have a better chance to kill him. This also works if you throw a Chaff Grenade at a enemy's radio.