Lists are stored to the '~/Documents/Maxprog/MaxBulk Mailer/Lists' folder.
The only way to empty that folder is to drop its contents to the trash or to delete the lists from inside MaxBulk. Anyway, MaxBulk documents also save lists so you can simply use last documents to restore them. Load a document, switch to the Recipients panel and use the 'Save list' menu from the list pull-down menu.
Stan