The odds are someone over the course of history tried that already. It gets to the point where the Borg aren't expecting an attack, but once they are attacked by something that they've encountered before they adapt.
Where our heroes got the Borg the first time is simple: The Borg were NOT expecting an attack from an external port on their network... they were expecting weapons fire from outside the cube. It's only when you catch them totally off guard that they can be defeated... and that trick might only work a few times until they catch on.
A good example from our RPG is a network of transporters beamed billions of gallons of water from a class-M planet into an orbiting cube. Did not destroy the cube outright but boggled it enough to give the players a chance to rescue the colony and escape. Had my players been more sober-ish they would have realized that the same principle could have been applied to sand. Or other solid matter. Ah well.
Borg work best as a force of nature, not a recurring bad-guy of the week... you either quickly write yourself into a defeat or you have to dumb down, and we all know which direction they took with 'em on screen.