Then, by using an access token you can send a curl request to delete the unwanted signature: curl -X DELETE -H "Authorization: Token token=token123-yourtokenhere123" "https://zammad.example.org/api/v1/signatures/<id number>"
Alternatively, use Signature.find(<id number>) in the console to check the signature and then Signature.find(<id number>).destroy to remove it.
In my local test instance I could not spot any immediate issues after doing this, but please check for yourself and you are on your own when running any of these commands/api calls, as the warning from the documentation says.