You can affect this property as though it were a linear list. So:
sprite(7).scriptInstanceList = []
clears all behaviors attached to a given sprite.
Also: for those using javascript, this syntax works:
sprite(7).scriptInstanceList.add(script("myBehavior"));
It seems to hang if you use the above code to initialize the list, but this worked for me:
sprite(7).scriptInstanceList.deleteOne(script("myBehavior"));