I've searched the forums and FAQs and I can't find the answer. Say a hero is grappled by a Naga, and weilding the Mace of Kellos. He spends enough surges to knock the naga back one space or more. Does this break the naga's grapple on the hero? Is this an acceptable way to break a grapple, and possibly the only way to do so?

Grapple just makes it so that adjacent enemies can't spend movement points. Using the Mace of Kellos doesn't require movement points, and knocking back a Naga generally means that you're no longer adjacent, so yes, that could end the Grapple.

The more common method is simply to kill the Naga.