How can I block Instant Messengers?

I administer the network for our school district. The students are abusing MSN Instant Messenger, AOL Instant Messenger, IRC chat, etc.
How can I block access to these types of sites?
Can I do it via the Windows Registry?
Can I use my SonicWALL to do it?
I thought I had found a Registry fix on the Internet, but after trying it, it did not work.
Any and all help would be greatly appreciated!
Thanks.