Some interesting ideas so far... some thoughts:
Simply make it so captured cities DONT produce supplies for like 1 month gametime
This only solves half the problem - yes, it makes it so that the "invasion" troops have a supply problem, but it still makes the invasion generally successful - the defending region will have it's production disrupted, cities lost, possibly even bases or a capital - in general, the attacker gets most of what they wanted.
Simply put a troop/unit limit on amount allowed to transit at a given time
This, and the other ideas about limiting the transit treaty (ie "keep moving") are difficult from a gameplay perspective (how does the player know what's going on?), and also limits certain situations (ie, alliances where the other player needs to come to the defense of the host player, meaning that a large troop movement in is actually desired).
I don't sense a lot of support for the 'surrender' option, but the efficiency/ammo hit option does keep the attacker from doing land captures or scortching - keeping the goal of "Dont make it so people cant try, make so people wont likely suceed. "
(In fact, with this option, the attacker can still get some success by bringing supply trucks or planes with his troops; this would resupply the troops after the attack starts, accomplishing some captures and distruption, but at the cost of low-efficiency unit losses etc).