What do you want to teach?
Some may think soap to the mouth is extreme; but you have to teach your kids that some behavior is extremely intolerable. My wife and I have reluctently used bar soap to clean up potty mouth after a couple of warnings to two of our four children who needed it. It worked extremely well! Children are not all alike so what some people consider extreme punishments are not required by every child. As I said only two of our children needed it because they had strong stuburn personalities.I can agree with the statement that children are not all alike, and that some behavior is extremely intolerable. For example, hurting someone is not to be tolerated, ever.
I find the image of holding down a child to put soap in their mouth so distasteful it turns my stomach. If my children can't count on me to not hurt them, who the heck can they count on?
How would you feel if this was done to you? I'd be furious, and would resolve, "well, I'll never let you see me doing that again! You can't control me!" It divides the parent and child, turning them against each other, OR squashes the child's will. This isn't generally considered good either, except for those who prefer sheep for children.
I, too, have a child with a very strong personality. (I don't label it 'stubborn' as that obfuscates and divides.) A master of her own ship, I call it. Yet, we got through her potty mouth stage without resorting to soap in the mouth. How, you ask? With humor, and perspective.
For perspective, I turn to the long run. What do I want to teach my child? Blind obedience in the form of "you will NOT say those words or you will be sorry"? Or, understanding the reason behind the preference, "no one wants to be around someone that keeps talking about things that happen in the bathroom! That's so gross!"
A child was singing an off-color ditty in the back of my van a few weeks back. My daughter giggled a bit, and I rolled my eyes, then the conversation turned to something else. She giggle because it was slightly funny, not because "mom is going to kill us", nor did she need to worry that I'd do something crazy like treat her friend with a bar of soap, nor was she a prude in danger of becoming the stick in the mud. The moment happened, it was over. It is not a hot button issue that she could push to get a reaction.
Contrast that to a child who was physically shut up by soap. The reason behind the rule wasn't understood (else the warning would have sufficed), and the self discipline to behave in a socially appropriate manner wasn't practiced. Soap squashed the behavior at the expense of the big picture.
Now if someone is belted at the playground, I want my children to be the first one there to say, "that is NOT OK!" Recognizing a situation that requires involvment is easier, I contend, when you don't have to deal with broad exceptions to the rule, such as "well, if a parent wants to hurt their own child to get quicker obedience, that's OK, but this case etc etc..."