Plausible deniability is an important part of bitmessage.
Messages are not only encrypted, but also signed with the senders address. This prevents strangers to claim to be a specific person but it also might cause trouble. if a message is decrypted and the signature is verified, the content of the message can be used against the sender, especially if his physical location is known.
Actions for plausible deniability
The simplest step is the deletation of the Address Block in keys.dat. It removes the address from the system so it can no longer be used to send or receive messages. With sophisticated data recovery methods (especially on modern file systems) the address block might be recoverable. If the address block is deleted and no messages are sent anymore from it it can make investigators suspicious.
A more efficient way is to make the specific address block public. The easiest way is to send it to a widely used Mailing List or DML. It allows the user to pretend, that this is a public address or that he never used it at all. If at least one user copies the address block to his keys.dat, the client starts accepting messages and answers messages.
This system is actively used on bitmessage.ch.
Wikipedia: Plausible deniability