Don't spit in trash cans and ashtrays.

This sentence is very concise. If you change the sentence slightly, you may not be able to convey the meaning. I suggest you make more efforts on the design (cartoon) and color of the signboard to make it cute and cartoon, not stiff.

If you have to change the words, "Make your phlegm as elegant as your speech" and "You are beautiful! Don't spit everywhere, it will ruin your image! "