There is no preprogrammed method of preventing subscribers from altering their subscription, but:

1) LISTSERV(R) has an "exit" feature which some programmer might be able to design a method;

2) you can implement the CHANGELOG list option and do a daily GET and look for recent activity;

3) you could design some method to rebuild the subscriber db everyday (that would be resource and procedure intensive)

Email delivery is inherently unpredictable -- I would suggest that you don't try to over design the LISTSERV(R) subscription process so much that you think things (processes) are perfect.

(The command exclusion thread has taken place several times before here.  You might want to review the archives for a deeper discussion.)

/Pete Weiss -- Penn State