Thanks for your reply Angimon,
We do not advertise our rates either, as we believe this cheapens the whole town not just our motel, and if you listen to other moteliers and brokers they will all say that sandwich boards are a scourge. We were also led to believe that there was an unwritten agreement/understanding that people wouldn't do this. Times are/have been tough but there are others ways to entice people into your motel.
As for the "thief in the night", the other motel owner who took the rego. plates asked the police to charge them with trespass but because they are not doing anything"wrong" there is nothing the police can do. A sign at the front of the motel saying "motel guests only past this point" or something of similar nature is a different story so they said. So put up the sign and then be be on the lookout for that 'no moral' thief!!