Three goals inside a five-minute period in the first half helped Crusaders keep up their pursuit of Premiership leaders and north Belfast rivals Cliftonville.
Gary McCutcheon thumped home the opener on 36 minutes and Paul Heatley fired in the second two minutes later.
Chris Morrow lobbed Dwayne Nelson from 40 yards on 40 minutes after the United goalkeeper had fluffed a clearance.
Colin Coates headed home powerfully on 60, McCutcheon netted on 69 and Alan Teggart flicked in a header afetr 79.
The first chance of the game fell to the visitors, but Alan Davidson's well-struck free-kick went over the bar.
At the other end, Declan Caddell headed over from a Gareth McKeown free-kick.
Heatley pulled the ball back for former Sky Blues striker McCutcheon to break the deadlock and then fired in the second after Nelson had saved from McCutcheon.
Morrow's impressive finish put the Crues in full control and skipper Coates got in on the act by heading in from Heatley's corner on the hour.
McCutcheon grabbed his second to extend the lead, before Teggart secured a late consolation with his header from Tony Kane's right-wing cross.
Stephen Baxter's side remain second in the table, nine points behind Cliftonville, while Ballymena lie eighth, with their hopes of a top-six place diminished by this defeat.