Maybe, but probably not as expensive as you launching it in the first place as a percentage of money involved. I'd bet there are a few old birds up there that are still under control, but not serving any particular purpose. Steer it into a collision course, and bam, your server just became a cloud of garbage orbiting the Earth.

On the other hand, that would seriously piss off the rest of the international community, especially if the two satellites were high enough to clutter up existing satellite traffic.