Answer: Short of patiently training the dogs or installing at least a temporary physical barrier or perhaps an "invisible fence", repellents would seem to be the only solution. There are a number of commercial products available, but they do need to be applied and reapplied according to the label instructions in order to be effective. Most large garden centers stock a variety of them. Perhaps one or another will work for you.
Here is a recipe for a home-made dog repellent which is intended to keep them from digging but might help in this case. Unfortunately it sounds rather smelly to use right next to your deck.
Mix into a quart of water: one clove garlic (chopped), one onion (chopped), 1 teaspoon Tabasco sauce, 1 Tablespoon cayenne pepper or chili powder, 1/2 teaspoon dishwashing liquid. Dribble over area to be protected. Be sure to wear rubber gloves and be careful not to get this in your eyes or on your skin.
I have one other thought. Is it possible that they are marking the area because of some wild animal living or sheltering under or near your deck? You might look carefully around the deck to see if there is evidence of any frequent or regular critter activity. (Woodchucks, for example, often set up house under decks.)
Good luck with your shrubs!
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