The powerful frontman took his tally for the season to 24 as the Pirates, who thrashed Hereford 6-1 at the Memorial Stadium earlier in the season, completed an emphatic double over their West Country rivals.
Hereford were under the cosh from the start as Rovers forced three corners in the first seven minutes.
Goalkeeper Peter Gulacsi was consistently in the action as the visitors rained in shot after shot.
Gulacsi kept his goal intact until the 26th minute. Rovers won a free kick on the edge of the penalty area and Lambert drove a shot through the defensive wall and into the net.
Lambert added a second, eight minutes later, brushing past Godwin Antwi to crash the ball home from 15 yards.
Hereford had little answer to the Rovers' threat although Kris Taylor had a long-range effort tipped over.
And things might have been even worse for the Bulls five minutes before the break when Craig Disley headed in only to see the effort disallowed for pushing.
Hereford needed an early goal in the second period and it so nearly came on 54 minutes.
Febian Brandy got away on the right and his cross was volleyed goalwards by Matt Done, with the ball striking an upright and going behind.
The action transferred almost immediately to the other end and Rovers were given the chance to kill the game off in somewhat dubious circumstances.
Hereford skipper Karl Broadhurst went in for an innocuous looking challenge on former Bull Ryan Green inside the area.
Referee Roger East awarded a penalty and Lambert cracked the ball home to complete his hat-trick.
After that Hereford had no way back and Rovers emerged as easy winners.