[Calchet]

It's generally a Very Bad Idea to send your pretender into the arena.

Until he dies, he will be forced to fight during each arena challenge, and unless he can change shape to something without hands, he'll be hanging on to that trident.

The only way to make him no longer be the champion is to let him die. If you just want to remove the trident, the Transformation alteration nature spell can likely turn him into a handless animal, but that's risky, and he'll probably become much weaker.