You could put down a sheet of of semi-permiable membrane over the soil and cover it with gravel. You still might get bits of grass seeding in the gravel (very little) but it will easily pull out as it wont be able to root properly.

Trouble is, Grandad, that the rock plants wont be able to find a way through either. I should think that this problem is common to all of us who grow rock plants - just keep on weeding :-(((

You can put slits in the membrane to put your plants in. Just cut a cross shape in, pull flaps over, put in plant and put flaps back and replace gravel.round plant

Plantex membrane does not stop grass growing. It either grows in the gravel and gets its roots through the membrane and is even harder to remove or the roots spread out widely so that when you pull it up you remove half the gravel too. I dislike the use of membrane for growing plants through.
And in any case if this is a rock garden then covering th erocks with emebrane might be a little difficult.
Having said that we do have a rock garden which was built on top of a sheet of the stuff, but that was to stop a mole from coming up through the rocks. It worked for that.
So back to the original question. As Bulbaholic says there is no easy answer other than to keep at it. Try to get the grass out before it goes to seed if you can. If there are big enough gaps between your rock garden plants then you could use a weedkiller like Roundup on the grass. Either paint it on or spray gently on a windless day. Hope you do not have the same grass as we do, it is the same colour as the gravel we use, almost impossible to see and it flowers at 2mm high!
Oh for the return of Weeout which was a grass killing weedkiller, no longer sold.
PS. If the gaps between plants are big enough you could use a gas flame gun. It does fry the seeds so you do not get as much grass next time.

I also have that annoying short grass in our rockeries, Owdboggy, very difficult to see sometimes.

You have my sympathy Guest as Bulba says you just have to keep weeding... trying to remove the tiny blades of grass is a pain!

Son-in-law has got membrane down in his garden, odd bit of grass will seed in gravel. Owdboggy said pull grass out before it goes to seed. Same principle with gravel, root dos'nt settle as quickly as in soil. I put membrane down for neghbour put gravel on top no problem with grass that was about 5yrs ago

I could not see how to use membrane in a rockery as soil and grit etc would settle on top as the plants spread and thats all you need for grass and weeds to start? I get mares tail and other weeds in my very plant full rockery, which in between the rocks is also covered with coloured slate and rock chips. I just take the weeds up as soon as I see them, they get less the more the plants take over and weed.

I don't think you can use membrane in a rock garden.. or, at least, I don't see how to and, given that we know some of the best growers in the Scottish Rock Garden Club and they accept the need to weed, I don't think you can. Rock/Alpine Gardening is not for someone who wants an easily maintained garden. You need to be constantly out there weeding, tidying and snipping (note ' snipping' not 'pruning'). Frankly a rock/alpine garden is labour intensive! And we love ours! If you view gardening/weeding/potting on/etc as fun then it is just that. If you view it as work then change your garden!