Villains don't have to play fair. "You throw up your water spell in a perfect circle around the blampire sorcerer. His grin lets the hair on the back of your neck stand up and he just stomps on the ground and your control over the water vanishes, the circle collapsing into a puddle on the ground."
*hands over fate point*As to running water as a supernatural barrier, I think it isn't to be taken quite this literal. I think the idea mostly comes from a time where traveling through a forest at night is dangerous and frightening. If you made it over the bridge, you were safe. Not so much because of the water, but because a bridge most likely means there is a settlement nearby or the bridge itself is guarded. Twist that through a few hundred years of folklore and you get the "running water is a barrier against the supernatural".
But taking that as a basis, I would think it makes naturally running water a barrier, not so much magically running water. So a spontaneous flow of water after a heavy rain could easily serve as a barrier. A river definitely. But just moving water would be a bit weak, I'd say. I would also think that even if it would work, you'd need tons of water. Literally. Even the rainstorm is far more than I would think a mortal wizard can move just like that.
However, wizards are sneaky. They cheat. Water magic might not be able to work as a barrier like that, but its entropic nature can still do a lot of good. Shutting a wizard out from his magic takes more than a mere circle though. You need something of him, something you can link to him. It's definitely not something you just whip up with an evocation.
If your wizard wants to just throw some evocation block on him and let the others attack while he's helpless: enchanted items. Let him create a big invulnerability crystal, much like Harry in... that one book. As soon as he notices his magic is gone, he throws up the crystal and is invulnerable until the wizard stops blocking him. This could, in effect, be another version of the compel.
Oh, another thing that's really fun is to just let the players kill the sorcerer as a pawn sacrifice and then come out with the
real threat. I did that in my pbp game here on the board. The players plan was quite cunning, they had a sniper up in a bell tower and the rest was going to keep the necromancer busy till he could take his shot. Though the necromancer had a demon bound to his magic and when he died, the demon took over. The fact that the players had the brilliant idea to lure the
necromancer onto a cemetery for their plan didn't make the situation any better. The words "oh shit" were uttered several times.
You can find the fight here, starting in the linked post:
http://www.jimbutcheronline.com/bb/index.php/topic,28668.msg1790316.html#msg1790316Oh, and used that there as well: minions. Lots and lots of minions. Maybe he's especially good at some sort of summoning. Maybe he's got his own personal ghoul army with him. Or some fellow black court vampires. A bat-like version of pixies. Elemental servants. Vines and tentacles. You can go wild with minions.