They're keeping the fe open forever? If not, check for a new version at startup and don't allow them to open the old one. If yes, call a function on every form/report open to check and if there's a new version, cancel the opening. Some developers require user to use desktop shortcut which just downloads an new copy regardless of whether or not it is updated.
The risky part of kicking out users is that they might be in the middle of an edit, which might corrupt data if you force a close. Could have a way to check that they're not using unauthorized copies also, by checking the application path upon startup.
EDIT - agree with WGM; this is not handled by the be.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.