My opponent is cheating/escaping/being annoying!
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Click to seek advice
First off, try not to get angry. Go is a wonderful game (but only a game still) and it would be a shame to ruin it for yourself by getting angry for one person being weird. We have many players from different cultures, different customs, and of different characters. Some are more passionate than others and can get frustrated when there is a sudden upset in the game.
First thing to do is to send the other player a PM. Be kind, ask nicely what’s up. Often times this solves the problem for both. Often there is just a misunderstanding.
If, however a PM does not help and you are still sure the other player is acting maliciously, let us know. We want this place as welcoming as possible and some people just do not have the self-control to be tolerated here. There is a “Call moderator” button on the right hand side slide out menu. Try to briefly but clearly describe the problem, so we do not have to ask and can act faster.
Be patient with us! The moderator team is run by volunteers and there is not someone present 24/7. Please do not spam the button several times. It will in fact make the reaction time slower for us. If no-one is showing up immediately and you are getting frustrated, just resign the game and play with someone nicer in the meantime. We will get to you eventually, annul the game, and punish the culprit. 
Do not let one angry guy ruin your day.