There are several things you can try my favorite is spraying a bitter spray like bitter apple or bitter herbs on the plants. This might help and it wouldn't hurt the dogs, plants or the environment.

http://www.ehow.com/how_2342324_keep-ones-dog-from-eating.html

You have many options, all of which are inexpensive and safe for your dogs:

1. Build a "box" around your garden. Use 2x12 boards to create a box around your garden, and then attach chicken wire to the boards to extend the height of the barrier by about 18-24". This will not only keep your dogs out, but can also keep rodents out as well (if you're sure to seal up the chicken wire well).

2. Use wire cages. You can get wire "cages" of all sizes from garden centers. Turn them upside down over your favorite plants. Again, this offers some rodent protection, too.

3. String bird netting around the plants and garden area. Use garden stakes to suspend the netting if you need to. Larger dogs may break through this if it is not secured well.

And don't forget to check with your veterinarian to make sure the plants aren't toxic to dogs. Many plants that are safe for even humans to consume can be very lethal to dogs (wild garlic and onions, for example). If your plants are endangering your dog(s), then you'll want to plant something new in the area that might not attract them as much.