Question:
I would like to know if it is possible to restrict the ability of a user to modify the members of a distribution list (we have users who are adding and removing members of our distribution list via the Address Book).

Answer:
You should check to see what kinds of permissions your users have on the Exchange server. By default users shouldn’t be able to modify any distribution lists through the Address Book unless they’re listed as owners of the list.

I suspect that somewhere you’ve given the Everyone (or a pretty large) group a much wider permission than you intended to.