Kirk should have added a second goal but skewed his header wide from a great position after another excellent cross from Valois.
St Johnstone continued to search for an opening, although Parkin was caught in an offside position with a close-range header in the 78th minute.
Michael Turner's lofted header inexplicably drew Watford keeper Richard Lee out of position, allowing Garcia to head towards goal and the veteran Barmby raced in to make sure.
Eleven minutes from time, Ibrahimovic -- who signed a five-year deal with Barcelona in 2009 but whose spell at the Nou Camp was so unhappy he left after just twelve months -- converted from close range, despite being in an offside position, as he reacted quickest to a towering header against the post by the outstanding Thiago Silva.
Visiting keeper Chris Day stopped a McConville header, while Stevenage midfielder Stacy Long shot wide from a good position in the opening half.
Striker Jawad Ouaddouch got a glancing header to a long range effort but looked to be in an offside position, setting off an invasion of officials and a fight between players.
The Ivory Coast international controlled a header with his first touch before striking a precise strike from a central position on the edge of the box that nestled into the bottom corner.
Bendtner was trotting back from an offside position when Walcott was released on goal by Johan Djourou's clearing header, but the Dane ended up colliding with Williamson as the centre-back tried to intercept the ball, leaving Walcott to dink over Krul to add to Arsenal's lead.
Hamilton's McArthur, who had run from an onside position, had the ball in the net after he nudged Joel Thomas's header past McGregor, but the goal was ruled offside.
When Diop muscled his way into a position to get a dangerous cross in, only for Brian McBride to misdirect his header, you could see why United may have felt vulnerable.
But the closest the First Division side came to a goal was when a powerful header from Alex Notman bounced off the crossbar - but it mattered little, because he was in an offside position.
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