The webhook uuid can be found at the end of the URI path. For example, the list endpoint returns an object with a URI key:

{ "uri": "https://api.calendly.com/webhook_subscriptions/AAAAAAAAAAAAAAAA", …. }

You can use the full URI, or extract the UUID which would be “AAAAAAAAAAAAAAAA” in the example above.

Alternatively, you can use a web UI (https://webhooks.calforce.pro/) to view / delete active webhooks.
 

Ok I found what I was doing wrong : I was passing webhook_uuid as part of a json data in the body of the DELETE request instead of just passing it in the url path :D

Thanks for the web ui link. Couldn’t find it in the docs, and was thinking using api calls to just create webhooks was quite overkill.