I had that problem with my Landy sail. A line through the grommet and remembering to cleat the in haul seems to prevent it. I think the cleating of the in hull is the important part -- it needs to resist sheet load and the little scrap of line I use for the grommet really isn't doing much except buying me a bit of time if/when I forget.

Other than that, I also thought about stitching some more cloth around the bolt rope at the bottom to make it a bit more snug.