I went on my realm and my friend was trolling me with /kill in command blocks and chat. He also kept teleporting to me so his head was inside mine, so I couldn't break his command blocks.
How can I stop him (without taking away his op)? Is it possible?
In the standard Minecraft, the only thing you can do, is to de-op him. You can't limit the commands, a user can execute otherwise. But the commandblocks which he already placed would still be a executed. You can break them by standing on them and using the following command: /setblock ~ ~-1~ air 0. This command deletes the block below your feet.
Mystery's answer is correct. I would like to add that this is possible in Bukkit though. Use a permissions manager to do so. I personally use McMyAdmin, since it also adds an interface for the less tech savvy server admins, but you can also use PermissionsEx. The plugin Essentials should give you the opportunity to disable command blocks for a single player.
Note: you did not specify a version. Some of these plugins may not work and require alternatives depending on your version.
Teleport go your friend, /kill him then break the command blocks. I don't play realms so I don't know much about it
