Vanilla 1.1.9 is a product of Lussumo. More Information: Documentation, Community Support.
Essentially the answer is "no, we can't do this, because we have no access to the software".
Options include:
One can of course opt out. Put your email address in your public profile, and clear the email address field. (Is keeping email addresses secret really a relevant concern these days? Mine has been prominently displayed on my web page in plain text for many years, and essentially no spam reaches my inbox.)
You could create a new email account, that would never be discovered by looking for md5 collisions, and set it to forward to your standard account.
I was unaware of Gravatar's terrible terms of service. (Aside: I don't remember ever particularly "signing up" to use Gravatar. But I do use it, on many websites, and had never thought much of it.) I agree with Dmitri that in a better world we would use (or, rather, allow to use) some other service. Dmitri: as you understand the issues better, I vote that you leave the desired change requests with SO.
(There are, of course, other security things to also dislike. E.g. it would be great to allow to use https. Can MO weigh on SE to fix these things?)
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