If you get a wizard within 18", you can try to stop them. Just roll 2D6, try to beat their roll. But heck, it's not like they can hide their caster, and you can snipe it ridiculously easy.

It really depends on the type of army you have. You could try a lot of missile fire, some units that have crushing charges (Ogres, Knights, etc.), casters of your own... Heck, if you have the models, join in and make it a summoning spree.