While the news Team Fortress 2 has gone free-to-play is good for most, for some long-time fans of the game, it's been seen as an assault on decency and common sense. Which they've reacted to by putting up a wall.
A script has been created by disgruntled fans which enables servers to filter out anyone trying to join a game using one of these new "free" copies of the game, restricting access only to those who bought and paid for a copy.
Before we go any further, note that this is far from a widespread issue. In fact, it's so far incredibly rare. It's also a user-made script, and has nothing to do with Valve.
But it does, in a roundabout way, have a purpose. While at first glance it seems to be an attempt by snobs to keep the new riff-raff from playing in their games, it's supposedly being employed by fans concerned that the free copies of the game allow cheats to easily return to servers after being banned, as they no longer have to buy a fresh copy.
That said, I'm sure there's at least one asshole out there actually using it just to be a snob.