That's called cheating.
Brrrr and Leonel Yunior do this. In real life, once both players pass, there is no returning to the game (this is standard go rules and applies in tournaments). On here, since there is the option to keep playing, since we are far apart from each other and can't talk much via the chat, people cheat by trying to win by forcing you to fill territory.
If this happens, hang in there, and post the person's name on here, so the rest of us know not to waste our time on them. I have a forum psot called "Brrr and Leonel Yunior, cheating in go"-- you could add cheaters' names to that.