If not detached, the network manager can be terminated by closing its Window with Alt+F4 (on Windows), or by pressing the Esc key (on Unix). You cannot do this while there are active connections. On Win32 platforms stopping the network manager does not stop the DP4 service as a whole, so it will not interfere with applications running locally unless the machine is running a Multiple Resilience combined client server configuration or the Named Connection Configuration.
On Win32 platforms you can stop the DP4 service, which will terminate all DP4 network connections and shutdown all loaded DP4 components cleanly. This works for any configuration of DP4 networking.
You can also shutdown a network client by using the DP4 shutdown program. However the shutdown program requires to be able to make a connection in order to work, so shutdown won't work if you cannot make a connection at all. (So on MS-DOS network clients you may have to reboot in this situation.)
On Unix DP4 network programs can be terminated with the kill command. You should not use kill -9, or at any rate you should first try a normal kill and wait for the network timeout period (10 seconds by default) to elapse before issuing kill -9. When DP4 is killed with kill -9 you must also use ipcrm to remove the IPC queues, semaphores and shared memory created by DP4.