Well, ritual spell combat doesn't stop armies from moving. It just generates combats during an earlier pre movement phase. Then movement is done and combats happen, unless something makes the move order invalid. So say it's summer and you ordered an army to move through a mountain pass. I would then cast wolven winter on either your province or the province you are marching to, making the connection broken. Your army should then not be able to pass, though I haven't tested this. There was also an example in the manual where a ritual is cast on a province that an army was moving through to get to their destination on the other side, and the movement is no longer processed since ownership flipped before the movement phase making it invalid.
I'll also say give the multiplayer a shot sometime, since I more or less stuck with singleplayer for the longest while, but I end up feeling there is a lot more depth that is brought out by human opponents, and drama, but the good kind, heh.
Not that a army in dominions can ever be "cut off" from supply (unless you got a commander with food items running around?) - supply lines would've been nice, with "Logistics" as a commander skill that sucks up supplys just like "administrator" does with castles, from provinces near him, with "well feed" armys (2x there supplys needs) getting more moral or something like that.
Fortresses do increase supply in relatively near provinces (maximum distance of four) based on Administration. I don't know how meaningful that is, but I suppose it can't hurt.
Baring access to supply items like endless bags of wine, a fortress is a valid way of making sure you can strike into 0 pop provinces. Supply also goes down during sieges, so there is in a sense supply lines, but they're also closer to ancient and medievel supply lines, more disorganized. Don't forget that part of supply is getting the resources and gold, and you can cut off provinces from a fortress as well.