If it were me, I would ban them, and let them come to you asking why. Or, if you know them, then I would ask them point blank about it. If they can explain it, great-- if not, heh.
Apparently, there are some though:
http://area51moh.com/A51-Army.htm (do a find for crosshair)
I do not know if this file even works-- and it may just be a legit attempt to mess around-- I don't see a real advantage to it This is why I would just confront them directly and ask for the file-- if they say, "gee, I can't remember where I got it", then just ask them to send the file, heh.