Such people do a lot of harm to the legitimate user of the ssn. Often these victims have to pay the abuser's income taxes or pay penalties for income that they are allegedly not reporting (simply because they aren't making it---someone else is). If you fear repercussions from the abuser or your boss, report the abuser to your boss anonymously. If the boss then does nothing, report the abuser to the social security administration and to local police (again, anonymously if you so choose).