I have started a thread long back on the same issue, you cannot do much on this. practically it can be really genunine or prob he might another tame guy who cannot accept the hard truth.
Its a request to chess dev team to add a feature which would show how many times the user disconnected the game in between, the higher the chances he is yet another sore looser.
I dont know if the warning that is shown is serious or just a joke."xxxxx may have violated our Fair Play policy - it has been noted and they may have their account restricted."
Sometimes the internet connection becomes unstable and that is understandable but other times it is just plain disgusting.
Whenever they are losing they just leave the clock running and waste others time. Its time admins did something about these fools who can't lose sportively