The Manchester City goalkeeper was angered after believing the Tigers man dived to win a penalty at the weekend, with the pair involved in a heated exchanged afterwards
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Hart appeared to move his head towards the 28-year-old before other players came to diffuse the situation.
The Football Association have yet to decide if action will be taken against the pair asa result of the altercation.
On accusations of Boyd spitting at Hart, Bruce said: "[Boyd] has tried to talk and shout and something's come out. That's what I've seen of it.
"George Boyd wouldn't do that, no chance. Not deliberately, anyway. He is as honest as they come - he was playing non-League six years ago."
In an incident-packed match, City captain Vincent Kompany could also face punishment after video footage showed the Belgium international kicking a wall following red card for hauling down Hull striker Nikica Jelavic.