Best way to keep grass out of rock garden

Bought a house a year ago that had a pebble garden in the back yard directly bordered by grass (no divider). Grass has started to intrude on the rocks and after a 10 day long vacation, I came back to find the grass had started growing over the rocks.

I'm guessing some grass killing herbicide would do the trick in the short term, but what are some things I can do going forward to keep the grass from intruding again?