If it’s hung you can kill the application from task manager and then restart the service.
You can find the file name to kill from editing the service properties in the services applet. The same file should be the file that you see in the task manager.
If you can’t find it you’ll need to reboot the server.
Some times this problem occurs when the service you are trying to stop is busy at the moment you want it to stop. I would recommend to check if scheduled client scans are not running, or some other client activity exists that has an open and busy session at the time of the problem.