As a kid my mom taught me that I had to go full HAL 9000 and say "I'm unable to do that, <name>" in a gentle but grave voice and then when they say "why?" I can drop "because I'm disabled".
When they ask I say "Because I don't have the capability to do that safely, and if I'm hurt, I will be hurt for many weeks, months, or years. Do you want to take the blame for my injury?"
Some look at me like I have three heads because not every disability is visible obviously.
Fine, gloves off: I then ask to borrow their hand, and I rub the ridge in my skull and explain my traumatic brain injury and the process of recovery.
I take a slightly sadistic joy in doing this, because I am giving the person in question a physically intimate experience they can never forget that will make them think twice about treating someone this way.
Almost always people back off when I ask to borrow their hand and I say "I'll show you".
Its a trick my mom learned from her mom, when she would explain about her back muscles and how some of them were just gone due to cancer, and how others were in permanent pain.
If you cross the boundary of language into something else, it overwhelms the other person and they almost always back down.
When it stops being abstract, it stops being something they decide they're in control of because it becomes real to them.
When they don't, other people socially pressure them to.
Its a dick move, but it works.
It has served me extremely well.
I do not like having to do it, but it works.
I am so glad I almost never need to do it anymore.
