Just had such problem in one daughter company where the licenses were used up. And - naturally the solution is quite simple.
You can download a freeware application from SysInternals webpage http://www.sysinternals.com
named "tcpview". This nice program displays all established TCP connections from your Native database server. Also it allows you to terminate the connection easily.
In order to do so, you have to run it on the server, but this should not be a problem for a administrator.
I added also a small piece of code to codeunit 1 to extract the computername from environment and connect it with session ID, so I know what computer I have to disconnect. I think there is also a way extracting the connecting port somehow, so also exact session in computer can be terminated.
Will this app be able to kill connections from clients that are not logged in? (i.e. shut down the client)
It can be tricky to understand what connection belongs to what application (if an user uses more than one of those). One way to do it is to record the actual port the client is using on application startup in database using information obtained from Netstat.