Go to the server on which WinAppxD and the APPX server processes run, and log in as Administrator. Bring up the NT task list, and look for an APPX.EXE listed with the process ID you want to kill (usually, this will be one given to you by the user, from their client's Windows title bar). Highlight it and click "End Task".

If another box pops up, saying that the application is not responding, click "End Task" there as well.

Hopefully, this will result in that process disappearing from the task list. If a process refuses to be killed in this way, it is time to take more drastic measures.

If you can't kill the process via the Task Manger, try using the NT Resource Kit's "kill" command. (You probably have this installed, since some utilities from it are required to install and run WinAppxD. Do a Find for "kill.exe" if you can't locate it.) The Resource Kit "kill" command is very effective, and using it is expected to kill the process even if the Task Manager method did not. (You must be logged in as Administrator to do this.)