@rperon , unfortunately this is not possible...
If you want to disable chat for only certain users, you can remove the checkbox "Enable User for Unified CM IM and Presence" in the End User page for those users in CUCM. But... this will not only remove Chat, but also Presence feature from those users.
If you want to preserve Presence status and disable Chat only, that can only be done for the whole cluster, not by end-user specifically. This can be done on the IMP server: CUPS -> Messaging -> Setting -> Enable instant messaging