An ascii-hole is a troll
, a button-pusher, someone who deliberately behaves like an anal sphincter
) safely behind the anonymity of the internet.
In real life they behave just like "normal people" and are well behaved, soft spoken, well-mannered and, often, highly educated.
"john" is such an ascii-hole, he came into "mary's" post about her anniversary and deliberately provoked her by calling her fat and a bitch.