When using Voyager you can't do without a password, though you can enter an empty password.
Altering or removing the password for TB can be done in just one way:
Create a internal backup from TB
Uninstall TB
Delete (or remove) the message base
Install TB again
Run TB
Now TB will recognize it's running for the first time and will ask whether to use On The Fly Encryption or not, to change the password you enable OTFE, to do without a password you disable it.
On the prompt for a new account or restoring an old
one you select to restore a backup.
Some notes:
As TB restores its backups to the original mail directory, you need to empty it.
Without a master password you can't have an encrypted message base.
You don't really need to uninstall TB, merely deleting the registry tree HKEY_CURRENT_USER\Software\RIT\The Bat! will do as well. And when you're running TB from a Windows restricted user account (as opposed to an admin account) deleting that registry tree is the only way, due to the way Windows handles registry issues.
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I'm just a user of The Bat! I don't work for Ritlabs.