SPAM started out meaning Unsolicited Email. THats wehre you get those advertisments in your mail, you know about porn and penis enlargement
But SPAM then spread to unnecessary/excessive advertising. In some online games i play(Star Wars Galaxies) soem people go to popular cities and shout out the location to thier shop. It gets annoying in our chat window to see it. So we tell them to "Stop Spamming our chat"