That is as easily answered as it is difficult. The easy answer is that what is considered cheesy and what is not is determined by the community as a whole. So the players themselves set the standards. But not everyone has the same opinion about every tactic. In the end the management decides on each seperate complaint whether it is cheesy or not.
The difficult thing is that we cant say what exactly is cheesy is and not. We are for example not going to say, 5 drop shots a set is cheesy. Besides that its impossible to keep score for the players involved it is impossible for us to check on it. We try to take the average opinion of the community and use it in our decisions. We try our best to make ITST matches a pleasurable experience for the people in the community without going over the top and set a million rules.
In practise this means that once we get a few complaints about a certain user and we feel these complaints are indeed considered cheesy by a large number of the community we ask the player to refrain from using such tactics in his future matches.
Welcome to ITST. I hope my explanation made sense.