The message is polite and professional (non emotional). It says "Welcome to washname, that the wash bays are for external vehicle washing only, any other activity are prohibited". I then list some of the prohibited activities such as dumping and garbage and that they are considered theft of services. I also indicate that I have a 30 camera video system that records every sq/ft of the property 24/7.
So aside from having canned messages for most situations, I also have them incrementing in severity. So if the people continue their actions, I fire more stern messages.
You need to be very careful using this as you do not want to accuse a paying customer who is about to wash. This is why I did not fire the message on their first visit.
I have described in the past how I do this, but basically I have a PC (in this case my DVR) that is hooked up to an map and I have outdoor speakers. So I log into the PC using remote control software (Like VNC or PC Anywhere), unmute my speakers and double click on the sound file I want to play. Its easy and cheap to do. The used amp was 50 bucks, the outdoor speakers another $50, a bit of wiring and the software is free (VNC).
Big