Alice was a heavy user of a type 1 anonymous remailer. She would post many messages to her favorite newsgroup, alt.fanclub.alice, and everyone would know they all came from Alice because they all bore the same pseudonym.
Assuming that the remailer worked correctly, Trudy could not impersonate Alice. After type 1 remailers werer all shut down Alice, switched to a cypherpunk remailer and started a new thread in her newsgroup. Devise a way for her to prevent Trudy from posting new messages to the newsgroup, impersonating Alice.