It tells you at the bottom of every digest how to unsubscribe. I get no feed back except to list the emails that have received the bulletin. I'll remove your email from the list for you.
Sounds like your email address isn't appreciated by the system.
To subscribe to a private forum there is a link in every private forum description where you can apply. To help you just go to
http://www.pprune.org/pub/private.shtml
There are no digest facilities for private forums and I am only running the digest on this forum on a day by day basis.
For all the details on how to make the most of the PPRuNe bulletin board go to the Frequently Asked Questions link near the top right of every page on the bulletin board (faq's) but again, just to help you and the hundreds of others who email me every day with the same questions here is an instant link:
http://www.pprune.org/ubb/NonCGI/faq.html
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Capt PPRuNe