Pretty sure you just go into arena defense, click them out, and either confirm or save.

you can't. you're required to have at least one champion in your arena defense at any given time.

You can't remove every single champion as you have to have some type of defense in there after you start. Otherwise people could get into Plat and then take out all units and since no units means you have nothing to fight means you won't be on the list. 

You always have to have something on defense. 

In case you mean remove all champions and have no defense at all, that's not an option. 

edit: I see I replied at the exact same time as Dae, oops

I suppose you could cancel your account. That is probably the only way. 

I rarely build up or use the one star dogs in the game, so I just popped one into each defense team. I started doing that because you have to have at least one, but also because I was past getting bronze  metals in the Arena before I had completed all the Great Hall stuff. Not having much of a defense put you back into the first level so you continue to get bronze metals instead of silver or gold metals. By the same token, using any four one star Champions works about the same as one star dogs, and if you don't want to upgrade your Vaults you can use those 16 spots to store new Champions. Of course, you can store any Champions there, but if they have rank and skills, they will fight for you whether you want them to or not.

Huh?

I did not think of that.