Go to the menu Tools > Account Settings and select the old account. Now click on the button Account Actions on the lower right and select Remove Account. You'll be prompted if you also want to remove all the data from the old account. Note that if you do remove an account, all the data of that account will become inaccessible, if the account was an IMAP account. For POP3 accounts, the messages will be kept under Local Folders. If you want to be able to still access the data from an old IMAP account, you can also tell Thunderbird to not check for new mail for that account anymore. Click on Server Settings fot that account and deselect the options "Check for new messages at startup", "Check for new messages every x minutes", and "Allow for immediate server notifications when messages arrive", and Thunderbird won't check for new messages for that account anymore. Of course when the old account is disabled, you won't be able to access the messages on the server anymore, so if you want to keep the data in that case, you'll have to copy it to Local Folders before the old account is disabled.