How to catch/block sniffers on my network?

hi all!

users on my network usualy use yahoo to communicate one with the other, lately it came to my attention that some of those users are using sniffers to detect passwords on our lan and yahoo conversations and such private info.

is there a way to block such sniffers?!
our topology has a low security level theres our private network- cisco 2811 router- internet.

any help would be appreciated..