Ok, here are steps:

1. click to account settings
2. highlight the usa.net account
3. slightly below in same column is 'Account Actions' menu, click that
4. now click the 'remove account' option. you will then see a screen with option to delete data, Do that. That causes account folders to be deleted. Since this is IMAP, they will be recreated when you recreate the account. Local Folders are not affected.
5. close and restart thunderbird
6. click help>troubleshootinginformation. that screen shows a matrix of accounts, titled 'Mail and News Accounts'. Usa.net should NOT be on list. This is just a confirmation
7. now, click hamburger menu in upper left and click 'create account'

I am getting ready to start your instructions but I don't see the hamburger menu or create account in step 7. Sorry, this may be due to my lack of experience.

Sorry, that term is used often for the little icon in upper right that consist of three short stacked lines.

Ugg. I followed your steps 1-6 twice. Here's what happened both times. After step 4, click remove account, the usa.net line and folders disappear from the left column. When I close Thunderbird and restart it, the usa.net line reappears along with the folder, "Inbox" (which is empty) and I get the same old window that wants my password. I click cancel to get rid of that window twice, because it appears a second time. Then when I go to help>troubleshooting info, "imap.postoffice.net:993" is still listed as an account under Mail and new accounts. So I have not bothered to create a new account per your step 7. Good grief.

On step 4, did you tick the box to remove data? that needs to be done to ensure folders are removed. I would also ckeck settings>privacy&security to verify that passwords are gone. I have never seen this for a usa.net account.

Yes, I checked the box and as I said earlier at the end of step 4, usa.net is no longer showing in the left column, just my local folders and my yahoo.com email account. I looked at the passwords and only yahoo shows up. So I am sure I have done as you instructed.

The only thing I want to be sure I don't lose at this point is my local folders. Do you think I could start a new Thunderbird account using my usa.net email and then move the local folders into the new account and just abandon this account and close it?