set suspend-on-exit

SET SUSPEND-ON-EXIT YES/no  -  whether the EXIT command suspends

With the usual YES setting, the EXIT command suspends the MM process.
  You return to the shell prompt, but the MM process still exists and you
  can return to it quickly with a fg command (the "mm" command is aliased 
  to the command "fg -s mm", meaning fg or start).

If you change it to NO, EXIT kills the process.  To run MM again, you
  would have to start a new MM process.  See the BYE command.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show
(at the MM> prompt:)  exit   bye