i just did that to see your army...you can't stop an army with scouts as far as I know, since movement in friendly territory occurs before attacks. Thats why I attacked both the provinces on our border, since I wasn't sure where you were going to go.
Edit:
Actually i guess I didn't know the movement rules as well as I thought. I thought you had to be adjacent to an enemy province to attack it, since I've never been able to attack otherwise, but apparently if you've got a move 3 commander it becomes possible, even if your troops are move 2.
So, sorry about blocking your army...i agree that it's pretty cheap, but that wasn't my intention.
incidentally, does teleporting/gating/faerie trod/whatever
onto an enemy army stop it as well?