why some dullards think mixed ancestry means mixed race

i am of mixed ancestry, european & british, but not mixed race obviously, and last week online in a comment, someone, some dullard! insinuated or implied i was mixed race...and i felt very angered by that mistake on her part and false perception.

my question really is, how should you respond to someone doing that? deliberately trying to upset me and cause offence.

i have nothing against mixed race people, only that i am not one of them