While not a sign per se, I know of a ministry which were selling different things with religious slogans on them. One particular one that caught my eye was a pen with the commandment, "Thou shalt not steal" written on it!

At work we do it a bit different. Me and my co-workers put radioactive stickers on our utility knives because they always seemed to walk away. We did a test. I had three out and only one had the sticker. Needless to say the only one not stolen/borrowed was the one with the sticker on it!

THe parking lot sign that I saw while I was on the road and I was trying to make a U-turn in it said "No truck turn around, unless your coming in for service".