Boyd had a heated exchange with the Manchester City goalkeeper in the second half of Hull's defeat at the KC Stadium on Saturday.
The incident occurred when Boyd went down inside the penalty area after Hart dived to claim the ball on the deck with the home side trailing by a goal.
Video footage appeared to suggest the Hull substitute spat in Hart's direction after the goalkeeper had reacted furiously to Boyd's fall.
Referee Lee Mason booked Hart for his part in the incident with Boyd, who later added to the controversy by insisting Hart "put his head in" and should have been sent off.
The FA will now await the referee's report before deciding whether further action is required over the incident from a match in which Manchester City skipper Vincent Kompany was also dismissed.
But Bruce refuted the spitting allegation, labelling it as "a load of rubbish" and adding: "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."