Hello, i found a really good farming spot for WOTLK but the mobs drop a quest item that can be looted non stop which fills the bags after sometime, how do i setup the bot so that it doesn't loot this quest item ?

Well the fastest solution would be a addon that destroys the items after you loot them.
Something like this: http://www.wowinterface.com/downloads/info9441-KarnisCrapFilter.html#other