PS Choosing the "cancel" option only turns it off temporarily. It bounces up again the next time I hit the "delete" button

Since the latest Thunderbird update I keep getting a message offering to purge all my deleted messages from the disc. I cannot "not see this message again". The only options are to cancel, proceed or automatically purge deleted messages. How can I cancel this message

Check this article to understand what compacting (or purging) is, and why it's needed. http://kb.mozillazine.org/Compacting_folders