Like most people i have a number of email accounts setup in Thunderbird. Whats really needed is a simple setting like a checkbox on the Account Settings page that would suspend completely all sending and receiving for the account without losing the password, emails or other important data.
this should prevent the account from doing any internet activity like contacting the server EVEN if the "Get all Messages" menu-button is used. In all cases it should just skip over that account if the checkbox is checked to disable it. Unchecking the box would resume normal behavior with no further input required of the user. e.g don't come back and ask  for the password again because like with a lot of people its long forgotten
I have a bit wordy because googling around there are all sorts of wacky suggestions like inserting a space in the user login name or deleting the password, none of which are acceptable, nor do they work properly.