I have run across a few hams using other people's call signs. In the NY metro area this is more common than you may think both on repeaters and HF. I have caught a few that gave themselves away after a while by saying something that proves they are not who they say they are or show no knowledge of the town they supposedly live in. Had one who forgot his last name once and another who forgot what city he initially told me he was transmitting from. Sometimes it is fun to trip them up with carefully worded questions until they disappear. When confronted they all vanish.
The funniest incident was someone who came on a local repeater and when pressed to provide his call sign by a group of us (since it seemed that he did not know what he was doing) he finally replied by saying they call him Big Charlie.
When asked why he was on the ham bands he simply said that he bought the HT from a friend used but the friend did not show him how to use it yet.
Not much to do about it other than get on the radio more. Most of these people use the internet to find their list of call signs and then listen and use the ones that they never hear on the air. There are plenty of those. No one in my region would get away with using my call for very long because I am very active on the radio. The expression use it or lose it may fit in these kind of cases.