The duo were involved in a heated exchange in the 69th minute of City's 2-0 win at the KC Stadium on Saturday, with Hart enraged after Boyd went down in the penalty area despite the England keeper making no contact with the midfielder.
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: "He (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.
