I don't think such an extension exists at this time.
If you already soft- or perma- deleted all their content, dropping them from the database using SQL or a tool like phpMyAdmin shouldn't cause any issue.
The fastest user-friendly way to do it is probably to use the FoF User Directory extension, so you can browse the list of users and click the 3-dot menu > Delete option of every user one after another.
If you have a list of emails/IDs in a text file or other database, you could write a script that hits the Flarum REST API to delete those specific users in a loop.