In order to keep dogs off of grass, it helps to create a visual boundary with a fence, a leash or with things that are unattractive to a dog. Engage a professional to teach a dog boundaries with help from a dog trainer in this free video on pet care and obedience training for dogs.
To keep a dog off grass, it's a good idea to have a visual boundary. Fence is very useful, and certainly, you could use a leash as well, to keep your dog away from a specific area. There are some detractants that you can put on grass or other vegetation, that are unattractive, in smell, or in taste to a dog. You could try those as well. I think you can get those from your local hardware store. You also might engage a professional, to help teach your dog boundaries, that this is how far they can go in each location, and this wouldn't include access onto the grass. For many dogs, grass is the most useful place, where they have learned to go to the bathroom, so if you're expecting your dog to stay off grass in a particular area, it's probably a good idea to provide them grass, that is acceptable, for their use. You might also take a look at your motivation, for keeping the dog off of grass. Is it because your dog is digging in an area? If that's the case, then you probably need to look at boredom. Do they have enough to do, mentally and physically? To keep them from wanting to engage in an activity, like digging. Also, destroying specific plants, might be your motivation, and in that case, you might be able to create a small barrier around that area, that will keep your dog away from it.