There are all kinds of ways of punishing someone, without causing pain

Don't allow them to close the bathroom door, or go there unattended. If they can't behave properly, they need to be treated like a child, and supervised.

Don't allow them to go pee in the toilet; make them stand naked in the bathtub, and then clean it and themselves up afterwards.

Make them stand in a corner - bending over at the waist, and holding their butt cheeks and labia lips open with their fingers. Inspect them every so often to make sure they are still holding the position.

Make them wear a loose butt plug when they go outside; with no panties or pantyhose.

For some, just walking around in public with no underwear on is punishment, especially if they are well-built.

Don't let them clean themselves off after sex; make them go out in public still smelling of sex. This works best if you can have sex in a semi-public place. Same sex couples can do it in a fitting room or something, and the sub can clean off the dom/me, but not themselves.

Embarrassment can be a fine punishment ...