I'm roaring laughing at your story!!!!
I've been in the same boat with my 6yo daughter!!!
We (semi) solved it when she was about your son's age - quarter fine for "no-no" words......from her or mommy or daddy. We're doing some renovations around the house - her piggy bank is growing by leaps and bounds :)
"We're doing some renovations around the house..."
Is she paying for them with her no-no money???? LOL!