A lot of work sure goes into teaching little people what excellence sounds and looks like in the classroom. This simple activity is just one piece of that. I've been doing this for years and apologize for not remembering the original source.
My kids helped me brainstorm ugly words. The only rule was the word couldn't be a swear word. We made our list and could have kept going (there are a lot of ugly words out there), but for sake of time, I called it good and said, "This is what I think of ugly words." I scrunched up the list and stomped on it until it was sufficiently flattened. Then I hung it at the front of the room on the whiteboard for all to see. (Yep, it will be there until June.)
"The next time you feel like an ugly word is on the tip of your tongue, swallow it." When an ugly word does appear though, I point to our smashed list and say, "That's an ugly word." The visual is pretty powerful actually. Sure this activity won't fix the problem, but it's an important piece, that in conjunction with all the others, moves us in the right direction to looking and sounding like excellence.