Monitoring and disconnecting users from an AirPort Extreme
Sadly, there doesn't seem to be a native way to do this.

With regard to detailed bandwidth monitoring, the Airport Extreme supports SNMP (Simple Network Management Protocol) so if you want to put in a bit of effort, you could set up Cacti http://www.cacti.net/ and point that at the base station.

As far as remote disconnection of users, there is no way that I know of short of the two options you mentioned.