Prerequisites: (1) it is not possible to delete a signer who has already signed the document; (2) it is not possible to exclude the only signer of the document - if this is the case, add the second signer first and then delete the first one.
Attention: the deletion is irreversible. If you want to reinsert the signer into the document, he will have a new token/URL to sign.
About webhooks: In some situations, the "doc_signed" webhook might fire after a signer is deleted. If this happens, the PDF signed_file has been updated.
Note: the request method is DELETE. If you use GET or POST it will not have the desired effect.