Consequences.

Every time you use the letter "u," I'm going to smack you on the nose. That's the consequence if you use that letter.

"But..."

SMACK

"Ow! Why are you..."

SMACK

"Aagh! Okay, I get it. Only it doesn't make any sense. Why does anyone care if I use..."

SMACK

"Stop it! God dammit, stop doing that! Yes, the enforcement is consistent, and yes, it's possible for me to comply. And no, there's no concept that I can't convey absent that letter, if I make an effort. Still, it's not necessary to enforce this restriction. It does no good and prevents no harm. As far as I can tell, y... the person I'm speaking to merely decided to do this. Why am I--and my nose--held responsible for that?"

"Oh, and I'm really getting sick of this, so... FUCK YOU!"

SMACK

You know what the problem is with kids these days? They just can't handle consequences.