I always try to be pro-labor but i had such a situation in my own company. A person I tried to make work for months, the situation just got worse the OP describes (it wasn't performance issues but more attitude/social/soft-skills related). I hesitated before firing them, fearing effects on team morale. After firing them morale improved immensely.

The best thing you could have done is fire them 6 months ago. The second best is firing them today. Don't hesitate.