There's a possibility.
Although I'm not 100% sure if the @noks command ables you to not hit him.
In your case though, make sure you take a screenshot of yourself summoning Thanatos with the fragments, I believe there's 4 of them. Once you've taken a screenshot of that, and someone tried to Kill Steal your Thanatos. You have the right to report that player for Kill Stealing your summoned Thanatos.