[@ReusableSword] the only thing i see wrong with the tactic is if you poison a water supply you can't make use of it yourself; And you risk it leaking into the water table, which the fort would most likely have shared with any camp near them. 

The injuries and lack of sleep related tactics would probably work well at driving off most grouos; And the use of fire would most likely chase off the rest, if only because of the attraction factor of the uunread. Both of those tactics are fairly easy to clean up and better then a semi permanent poisoned water supply.