Even if you were able to do that (which you aren't), it'll be a one-trick pony. A good summoning army would have a lot of redundancy. For example, a vampire counts summoning list at 2K would have 4 vampires (possibly 1 lord), each able to cast Invocation of Nehek. You erase 1 and 3 are still able to summon undead.
I play vampires as my primary tournament army and I agree with this in part. There will be a lot of redundancy in a VC Rez army- it's part of what makes it a good list. However, there are still advantages to knocking out 1 of his casters ability to cast the spell:
1) incoming dice- I suggested this in my first post, but if he's got 9 dice casting IoNs repeatedly, and you knock out a caster's chance of casting, now he's down to 7 dice. It's easier to stop. That other caster might still be tossing spells around, but vampires have very few aggressive spells that can be cast without a lord. If they have a lord, stopping him will remove SEVERAL dice (potentially 6) from their pool.
2) Not all vampires can raise regiments beyond starting their number of models. Some lists that go rez heavy field smaller regiments to start with, and pay 15pts per vampire to gain the ability to make those regiments bigger through IoN. If you stop a vampire who can super-size a regiment, you may have very well crippled his plans. For example, my general has 6PD and is one of only 2 vampires who can raise my Ghouls beyond their starting values. If she can't do this, I'm stuck with TINY regiments of 15 ghouls. I only cast 3 spells, and they're all easy one: Raise Dead, IoN, and VanHels Danse Macabre. They're the only spells we need, and they don't hurt anybody.
3) Redundancy- Vampire set up for Raise-specific lists are quite pricey. Without weapons or armor or magic gear you're looking at 145pts base. I well tuned army will try to field only as many vampires as they need. Therefore, if he's got 4 vampires set for raising, and you take one out, you've just put a lot of strain on his army. Also, IoN has a short range of just 12". If you knock out a caster, you've destroyed part of the chain that lets them keep their army going. Again with redundancy, they can probably mitigate this by overlapping bubbles, but still- 12" is a big distance in Fantasy.
Knocking out spells is still a viable tactic, but it only has a 50% chance of working, at best. You can't build a plan around it, because of the redundancy issues and also the poor odds, but you can certainly use it to your advantage.
Same goes for killing vampires. Highelves should NEVER target weapons at VC infantry choices unless they can reasonably kill a vampire. Why- because we have asf, hit on 3's, and wound on 4s, with more attacks than they'll ever get. Our basic infantry can demolish VC regiments. Vampires are the only thing that keep their army in the fight and make them so nasty. Aim for their flanking regiments and knights (who are dangerous or destroy your SCR), then vampires who you can catch alone, then Corpse Carts, and finally everything else.