Obsidian doesn’t really point at a folder. Any folder can be made into a vault and maybe you’ve done that with your Dropbox folder. In which case there ought to be an Obsidian file at its root.
My suggestions stand. Set up new vaults outside Dropbox. Open them and close the others. You should then be able to delete the original vaults from the vault selector. If you worry this will delete the folder, test it with a dummy vault. I believe it simply deletes it as a vault.
For Obsidian not to recognise a folder as a vault, you need to delete the Obsidian files in the folder. You may also have to delete a main settings folder.