Are there any other abilities that can be used, other than to KO the user, avoid being attacked by them, and avoid attacking them (dealing damage)?
There are three ways to end rage. The ways you end rage are explicitly detailed in the PHB
It ends early if you are knocked unconscious or if your turn ends and you haven’t attacked a creature since your last turn or taken damage since then.
And that is the only way, per the rules.
Mechanically, grapple doesn't work because he can still attack you, and restraining him also doesn't work because that condition still allows him to attack. You can knock someone out by reducing their hit points to 0 as well, and a melee attacker can choose to leave them stabilized and unconscious.
Your only options are not something that is a "move" or ability you can do, you would just have to prevent him from attacking something, which could be as easy as taking the dash action away from him or as complicated as tying him up and taking away his ability to make an attack. Get creative, you can do more than the book says you can do.
Bottom line is, you can't really stop a barbarian from raging. It's what he does. Other classes excel at other things, like getting free hitpoints from turning into a bear, or being able to cast magic, or being really sneaky. The barbarian is good at hitting things and taking a beating. He can only do it so many times per day and taking that away from him is like taking away a class feature. You can probably do it, but it's just not worth it and not that big of a problem.