Sorry that it doesn't provide a way to delete multiple users currently and we'll improve it soon.
If you use Google Workspace or Azure AD to sync users, you can sync again with the target group and do not select 'Keep previous users'. It will sync the users you want and remove other users except Owner and Admins.
If you use other ways to import users, please let us know if you want to remove all existing users except Owner. We'll remove them from our side. If you only want to remove part of the users, please go to myViewBoard support website and submit the user account list you want to remove.
Hope these can help