Stewart Maxwell’s side had no answer to the Scottish Junior Cup holders who, on second half pressure, could have won by an even bigger margin.
Rabs had an early chance when Willie Sawyers sent Shaun Fraser through on goal but Beith keeper Stephen Grindlay made a fine block from the midfielder.
At the other end David McGowan was off target with a powerful drive.
Play developed into a midfield battle and with both defences dominating in the opening period chances were few.
Rob Roy keeper Jordan Brown did well to snatch the ball as Ross McPherson approached, before Rabs were awarded a free-kick at the edge of Beith’s penalty box but Gavin Mackie sent it well wide.
Rabs were playing good football but their final touch was lacking and they were failing to trouble the Beith defence.
There was a scare for Rabs in the 37th minute when a angular shot from McPherson rattled of the top of the crossbar before being scrambled away for a corner kick.
Just as it looked like both sides were to go in at the interval on level terms, Beith took a 41st minute lead.
Darren Christie broke in from the right before passing the ball to McPherson who shot past Brown from close range.
Beith began the second half aggressively with McPherson then Mark McLaughlin both sending efforts off target.
In a Rabs attack MacDonald was forced to concede a corner before Fraser could make contact. From the kick Fraser shot past.
Rabs were dealt another setback in the 71st minute when a Reece Pearson defensive error allowed McPherson the chance to ram the ball into the net to double his side’s lead.
With Beith now well on top, and never looking threatened, they pushed forward looking to increase their lead.
Substitute Kenny McLean sent a volley across the Rabs goalmouth before Brown made a fine save from a Paul Frize shot.
Minutes later the Rabs keeper made a brave save at the feet of McLean then a last ditch tackle by Craig Buchanan prevented Christie from adding to Beith’s tally.
In stoppage time Brown again made a fine blocking save from McLean with Buchanan heading the rebound off the Rabs line as Beith ran out worthy winners.