Resolution:
Upon startup RTmonGUI subscribes to:
  /_machine_name
  /_mon
  /_all
  /unique_subject (RTmon is an RTclient).

To unsubscribe to the first three enter the following in your rtmon.cm file:

unsubscribe /_all /_mon /_&ltmachine_name>

where &ltmachine_name> is replaced with the hostname of your computer)

Notice that this statement uses the unsubscribe command, not the setopt command.

Note that an RTclient must always subscribe to its own  &ltunique_subject>.