I had already done that and the deleting API endpoint is the one I have been using, which the same one for both actions (delete only and delete&block) — the only difference is on the parameters that are used with it (block_ip, block_email, …) that I mentioned aboved.
I think I finally got around what was the issue with my requests: Discourse’s API wants the strings ‘true’ and ‘false’ instead truthy/falsy values. My bad for not spotting the notice about that on the docs.
That was probably what has been causing all this mess.