I think it should be made illegal to contact any employer in an effort to get one of their employees fired or reprimanded based on something you saw about them online. It should also be made illegal to fire an employee based on something they did online or in their private time, as long as it was legal. It should be considered wrongful termination.
The Trayvon Martin thing was an enormous story in the news for over a year. It is completely natural for it to show up in a few peoples' Halloween costumes around the nation. You can argue that it's insensitive or ill advised, and that might be so, but there's a pretty strong tradition of people trying to deliberately go for offensive costumes to get a reaction at some party or something. This kind of thing happens a lot particularly among the youngish and those who drink a lot.
It may not be humanity at it's finest moment, but anyone losing their job or being harassed over it is a far bigger problem than the costumes themselves.