I'm quite surprised that Amethyst creates a reaction on the user's behalf; I really don't see the utility and, as you explained, this action can have severe consequences.
I didn't know about this Amethyst automatism since I don't usually report using kind 1984, exactly to avoid being linked to questionable content.
I think Tagr-bot (npub12m2…e68j) is a good solution.
Finally, if you really must choose a negative reaction, why not use “-” as NIP-25 requires? At least clients would hopefully not show these notes as pinned content in the user's profile.