On my Mac mailboxes won't delete after Monterey update

[VivM]

First open a Finder window. You can use [command] [tab] to see active progs, and finder will be one of them. Then when you are in Finder use the file tab to open a new finder window. In the left side of the finder window you will see a column of Favorites. In that listing will be a folder that has an image of a house on it, on mine it has my name. That's your account. Click that home folder and you will see a listing of folders, one of them being Library. Click Library and in that folder look for the Mail folder. Click the mail folder and you will see a V9 folder. One of the folders in the V9 folder will contain your import# folders. I deleted all the numbered import folders and left only the import no number folder.
Now go back into mail and you will be able to delete the import# folders and their contents. You may need to restart mail. Like I said I also updated my IOS at this time so it may have helped. Good Luck

Thank you, shooskater, for your detailed explanation of what to do. I just followed these instructions today, slightly different setup in my case, and by deleting the import# files in V9 I was finally able to delete the 200 import mailboxes from Mail, altogether around 600,000 emails. Hooray! I didn't have to delete any mail accounts. So let's see how it all goes from here, they haven't come back yet today. I am very grateful for your solution, and everyone else's input. So simple in the end.