Dogs tend to sniff around before they relieve themselves. If you sprinkle some paprika or ground pepper in the area, it'll get up their nose, and will deter them. It washes away with rain, and the deterent might not be immediate, so you may have to reapply. (We bought a big container in the cash & carry, but depending on the size of the area, a regular sized packet should be fine.)
However, it completely stopped cats coming into our garden, and even though we stopped putting it down a few years ago, we still don't have them coming in.