You should use DNS as it was meant to be used - that is by using it with only correct information. Trying to obtain security by misconfiguring something is ill-conceived and (as most said) easily worked around.

Your best bet is to black the IM port with IPTables (takes about five minutes to set up if you don't know how) or monitor who is using the IM ports and clamp down on the user.

Still another option would be to disallow the application on the desktop either with profiles and roles or by walking around and manually removing it. You could even enable remote administration on your internal network and remove all the IM software from your desk.

But misconfiguring DNS is the sign of a bad administrator and a worse boss if he'll let you get away with it.