Sometimes Apple Remote Desktop hangs on the remote computer and/or you get disconnected.  When that happens you can get the message “can not control remote computer because it is running Apple Remote Desktop or another administrative application”.  To “fix” this …

With ARD on the local computer, Send the Unix command:

ps -A

This will a list of processes which are running on the remote computer.  Find the process number, the PID, for the Apple Remote Desktop application process and then give the command (send a unix command):  kill -KILL “PID#”  (change “PID#” to the actual pid #)

WARNING – the kill -KILL command is not kind.  It will stop any process without additional double check and, therefore, if you kill the wrong PID you can cause the system to halt and become completely unresponsive.

as in:

kill -KILL 25204