Well you can start by reducing their
acquisition range. No matter how far their sight is, they have shared sight and when you aggro one, the others can see you and will attack.
If you don't want to change all that manually, you can set up a couple triggers to Pick Every Unit and whatnot, setting their acquisition range.
Secondly, there are a number of gameplay constants that can play a part here.
Creeps - Guard Distance, Creeps - Guard Return Distance and
Creeps - Guard Return Time (sec) all affect default computer AI behavior.
I'm going to hazard a bit of a guess here...Guard Distance is the radius in which a Creep can be considered to be guarding his "Lair", and based on that assumption, Guard Return Distance is how far the Creep can go before he must return to the Lair and Guard Return Time is how far the Creep can be out of the Guard Distance but within the Return Distance, before having to return to the Lair. Make sense? (It's actually a guess but you get the point, those 3 affect default AI.)
Combat - Call For Help Range is another important one I believe (the others may not matter...).
Do note that changing stuff in World Editor to 0 can sometimes result in the "default" value. I'm not sure if that applies in Gameplay Constants but you would do to simply set it to a low value...
Try resetting all the constants and simply fooling with Acquisition Range first, though
