my buddy who is a an OSX guy but not a Maya guy says you should be able to stop the process via the terminal in OSX.
open terminal and type "top"
this will list current OSX processes currently running
To stop a process use the "sudo" command
Look for the "PID" number of the stalled process.
Open a new terminal window so that you can continue to look at the "top" window.
in the command prompt of the new window type. kill -9 228 or what ever PID number the stalled process has.
The process will be killed.
if that doesn't kill the process, then he can use sudo kill -9 228 to force kill the process.
You must be careful when using SUDO because the computer will never question you and you can inadvertantly stop something you shouldn't?