first you need to find out why your dog is digging? Is it because it feels good to dig? Is it because your dog is bored? Some dogs will dig because it is hot out and they want to feel cooler.
think about why they might be doing it. IF it is because it feels good you might wish to dedicate a certain area of the yard for them to dig and teach them to use that. If it is because they are bored, find alternate activities that will spend some of that energy both physically and mentally. If it is for coolness, give dog a pool and some shade to use instead.
I already have a pool so I don't think that's it. I think she's el boredo. I just did a quick search on diggers and found dog poop in the holes should work. I know it will. I just forgot about it cuz that's what I used to do with my brother's dog who used to dig to TimBucToo... throw poop in the holes. I can't believe I forgot that!
I will let DH know and if he catches her, I'll have him throw a ball too her instead. Maybe she'll stop.
The best solution I've read about for digging is to make a designated digging pit (sand box), and randomly bury toys, bones, other fun things in the sand box. Digging in the box becomes so reinforcing that they don't usually try to dig anywhere else.
The more aversive solution I've read about involves burying a full water balloon in the holes they dug and then covering it up with a little dirt. They get so startled by the balloon popping when they dig that they supposedly stop the behavior. I'm not sure if it works or not, but I've heard about this technique a few times.