In Thor: Ragnarok most prisoners, Thor included had one. Why not the Hulk? How were they able to make him submit and even get a haircut?
By the time we see Hulk in Thor: Ragnarok he has become the Grandmaster’s top champion and has been winning fights for years. He is clearly loving it and doesn’t want to leave, he plays up for the crowd and is having a lot of fun. It’s made clear he doesn’t want to leave when Thor starts coming up with an escape plan and Hulk doesn’t want to go along with it:
Hulk: Hulk Stay. [...] Thor go. Hulk stay. [...] Thor home.
The obedience disk might not have even taken to Hulk either, remember how Hulk spat out the bullet when Banner tried to commit suicide.
Bruce Banner: I got low. I didn't see an end. So, I put a bullet in my mouth...and the other guy spit it out!
So how did they get him to stay? Remember Hulk is quite a young character in mind: it’s relatively easy to persuade him to do things and he loves the simple stuff. Give him a chance to beat up a load of people and become popular and he’ll jump at it. The other reason he didn’t want to go home was because people back home didn’t like him. Those on Sakaar love and worship him.
Thor: If you help me get back to Asgard, I can help you get back to Earth.
Hulk: Earth hate Hulk.
Of course we don’t know how they got Hulk to submit and go along with the plan but that seems like the most likely option: certainly after the first fight and he gets a taste for people liking him. You have to remember at the end of Avengers: Age of Ultron Hulk isn’t just the mindless smashing machine we’ve seen before he has started to calm down already.