There is already a 4 years old (!) bug report that deleting of signatures is not possible via Webinterface delete signatures by the admin · Issue #2360 · zammad/zammad · GitHub
In the admin console guide Console — Zammad documentation I also can’t find how to delete signatures, but I think this should be possible.
So how to delete a signature via admin console?
I had a quick look, you can see what signatures are available by opening https://zammad.example.org/api/v1/signatures.json, and check individual IDs by visiting
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>"
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.