United were 2-0 up at the break, their first-half goals coming from James Pinder and Ollie Milner.
Pinder then notched after the interval to put Leigh Franks' side 3-0 up.
Seamer hit back through player-boss Tommy Adams, but United had done enough to seal the win.
Franks singled out Ben Briggs as his side's man of the match.
Goalscorer Adams and goalkeeper Jordan Wood shared Seamer's man of the match award.