I am not able to delete publishers from the UI and there is no option for deleting via the tfx-cli either.
No options in the UI in marketplace and tried this answer but there is no such command in the cli.
See here.
For example, you need a user that have permission like 'Owner' in marketplace 'publisher' instance.
You need to generate a Personal Access Token on DevOps side that have these permission:
Or you can simply select the 'Full access' section of the 'Scopes'.
After that, install the tool 'vsce':
npm install --global vsce
After the tool installed successfully, run this command:
vsce delete-publisher <publisher ID>
After give the personal access token and confirmation, the publisher will delete successfully:
Notice:
The publisher ID is here: