If you think your opponent is fighting dirty then you can review the entire game after it is over. Perhaps he was cheating, but this is almost never the case. Much more likely, you will learn a perfectly good strategy that you can use on your next game. In this case, "fighting dirty" is more akin to "geurilla warfare" in that it isn't a cheat or exploit, so much as a high-damage, low-cost attack reliant on either Stealth or simply a mind game of what your opponent is expecting, and what he isn't.