AMT does not really help in this situation, as you need the laptop to connect to a network you can host something on before it can receive an update.
Probably the best thing would be to run an agent handler in your DMZ so external computers on the internet can still get policy updates even when they are not connected to your network - then you can disable the user and they will get locked out even if they are just surfing.
In combination with SafeBoot suggestion, you could use the Endpoint Encryption Out Of Band management feature to change a users password. This request will be processed only during pre-boot, so if machine were left in preboot long enough for it to dial back it would lock the user out as well.
Hope that helps.