I'm a DM and one of my party members wants to use it to be able to walk without legs so it's just armor carrying a body (were starting at 5th level if your wondering). I want a way to make this a disadvantage and basically render him useless for a battle or two for fun. I know it says that The armor attaches to you and can’t be removed against your will but i was wondering if there was a spell or some condition that would get rid of the magic on it or something in 5e, thanks.
This doesn’t sound like much fun.
It sounds like this character is trying to have a bit of flavorful fun with their character’s abilities. From what I can tell, what they are doing isn’t affecting the mechanics of the game at all. It sounds like they are just trying to add some fun flavor to their character.
Why in the world would you punish this?
If you feel that this sort of RPing with the class feature is not serious enough tone-wise for your game, this is an issue that should be discussed out of game, not by ruining their fun in game.
I haven’t bothered to see if this is possible within the rules, because it shouldn’t be done at all. I cannot in good conscience equip you to take away someone else’s fun.
The armor attaches to you and can’t be removed against your will. It also expands to cover your entire body, although you can retract or deploy the helmet as a bonus action. The armor replaces any missing limbs, functioning identically to a limb it replaces.
That's pretty cut and dry. You cannot remove it against their will. Enchantment magic of some sort may be able to force them to remove it--but that certainly wouldn't be a "reasonable request" for anything like Suggestion in combat. You'd need something like Command or Dominate Person. (see: many, many cautionary commentaries on subverting the will of player characters)
And even if you did somehow get it off of them
You can doff or don the armor as an action.
All you have done is created a 1 action tax for them to put it back on.
You can't even shut the armor down in an Antimagic Field. The description of the armor does not say that the armor becomes Magical...and, importantly, you can Infuse your Arcane Armor, but you cannot infuse magic items.
Thus, Arcane Armor does not count as a magic item, and thus would continue to function in an Antimagic field.
A cautionary note
Make sure you know your player and that they are going to actually still have fun if you start trying to sideline them. They've already taken on the issue that they must wear their armor at all times or be crippled--which could easily be a social issue. I mean...how is the character supposed to go to a fancy ball while wearing a suit of armor...the "it fits under your clothes" feature of the Infiltrator model was removed when it went from UA to the final version.
The armor attaches to you and can’t be removed against your will.
The rule is pretty black and white. However the answer is there as well. It cannot be removed against your will. Arguably some magical compulsions might or might not change your will. However, under any definition old fashioned trickery works.
Consider if there were a cursed +1 plate armor, that once you attempt to don it, it turns into an animated armor and attacks! This would be a bane to any plate wearer (as donning their new armor first requires doffing their old armor). However for your particular use it could be especially troublesome if the animated armor first action is to kick away the old armor.
As others have mentioned however, the "fun" of exploiting a character physical disability would likely be questioned at many tables. It could be considered distasteful for everyone involved.