I've used the "Report abuse" feature on GitHub for such situations. Most of the time I see the user suspended, and the associated comments deleted.
Our GitHub admins can delete comments too.

On Tue, Jul 2, 2019, 1:42 AM Victor Stinner <vstinner@redhat.com> wrote:
Hi,

Sometimes, I get an email notification about strange comments (unrelated
or make no sense) on commits made 6 months ago if not longer. Usually, I
go to the user profile page and I click on "Block or report user":
"Report abuse". I'm not sure what happens in this case. I never checks
if these strange comments are removed by GitHub.

Victor

Le 02/07/2019 à 03:26, Mariatta a écrit :
> I've reported the user to GitHub. Ticket ID: 297185
>
> On Mon, Jul 1, 2019, 5:40 PM Guido van Rossum <guido@python.org
> <mailto:guido@python.org>> wrote:
>
>     There's a user on GitHub ('experimentalles') who has created random
>     PRs on several projects (e.g.
>     https://github.com/python/peps/pull/1116). Can we ban them? The PRs
>     look the work of a vandal or a bot, not of a clueless human.
>
>     --
>     --Guido van Rossum (python.org/~guido <http://python.org/~guido>)
>     /Pronouns: he/him/his //(why is my pronoun here?)/
>     <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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