The sentence is too direct and impolite. In English, you need to soften your language, especially when you are talking to customers. It is also better to use "do not" rather than the contraction "don't". I would also agree with putting the word "warning" in front of the sentence. I would therefore recommend the following: "WARNING: For your safety please do not leave clothes on the heater" Dime si quieres que explicatelo en castellano.

Overall this sentence is correct, everybody can understand. You can shorten it to say " For Your Safety Please do not leave your clothes on the room's heater to dry"

All are correct. Another way to say it could be: "For your safety and ours, please do not leave clothes on the heater to dry"

For everyone's safety, kindly don’t leave clothes to dry on the room’s heater WARNING : Don't use the room heater to dry clothes

You could also write, "Warning: For the safety of our guests and employees, we kind ask that you do not place your clothes on the room heaters as this is a fire hazard"