Soap in Mouth

Someone has been running their mouth lately and needs to be punished more than just a spanking.

I remember as a child my mom would put soap in my mouth and it was much worst punishment. I was looking to do this to my partner (with their consent of course). I was looking for a recommendation it seems like most the soaps are toxic.

Also if you have done it how long in the mouth do you do anything to prep what protocols?