Set an upper limit on the number of concurrent searches you will permit to occur in a given period of time. This is a pure performance issue that you have hopefully already considered. It is not normal for a surfer to launch a dozen searches in a second. Perhaps one every ten or twenty seconds is a more reasonable rate. Restrict repetitive same-criteria searches.
Real surfers conduct multiple searches on different criteria most of the time.
Establish a means to locally sever the connection from a Floodnet user. Consider using strike-back software to disable the attacker s browser and Floodnet software. Floodnet attacks are disruptive, not destructive in the classic sense of the word. But maybe that s the whole point. Use it to be forewarned of attacks, targets and methods.