In Account Settings there should be a tick-box for enabling/disabling each account. There are times you want to disable (ie switch to offline) an account temporarily until, say, you fix a problem. Not being able to do so means you get annoying error messages. Even if you disable periodic checking and checking at startup, TB will still try to connect to the server if you select one of the account's folders. A switch would disable the account completely. Disabled accounts could appear greyed out, crossed out etc in the accounts/folders tree.