The whole idea of "counting PRs" as a vanity metric is flawed because vanity metrics are flawed.
I don't think that there is a technical solution to be found here, as the problem is anything but technical.
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A hack/trap:
Comment "Ah yes thanks a lot for the hint :)", then make the changes yourself.
Then see how the person reacts to that.
Hack the grifters. Hack the planet.
I have noticed a lot of my Github followers who are either newer students or just new to Github have an immaculate green board of commits on their profile page, but it's so over the top I only like the ones that have creative illustrations or just very limited edits. Some seem so automated that it's hard to know how much they actually work on it. Others even are programmed to follow accounts then drop off once the person doesn't reciprocate, which is technically better than them staying but also systematically checking what sticks (also Github should limit some of these automated follows).
Or just pull the pr into your local git, git amend --reset-author, and then push it to master yourself.
My solution is to ask them for proof of work. Many of these disappear after opening a PR that they never tested. How can you claim to have fixed a visual issue without a screenshot? Insta-close.