This is not a hack. LimeWire will report that a file was not found when a freeloader tries to download a file from your computer. Wether or not someone is considered a freeloader is determined by your settings. Go to Tools > Options. On the Advanced tab there is a section called Connection Preferencing. Here, you can say how many files a person needs to be sharing in order to NOT be considered a freeloader. In other words, this is the cutoff minimum. There is also a slider that determines how much grace you wish to bestow upon a freeloader. Even though you may have a 500 file minimum requirement, if the slider is set to "Always", freeloaders will still be able to suck up your upload slots and bandwidth with ease.