When we were younger and dumber a girlfriend and I indulged this guy, told him he we were together, etc... he said his tongue was getting hard just thinking about it. It was pretty lame.
I have a set of wedding rings from an ex girlfriend that I still wear... when a guy is interested I flash the rings and say I am married. It is much better than telling the truth and watching his eyes glaze over as he imagines himself in the middle.