Hi Francesca,
A couple of suggestions:
1) There are motion sensor lawn ornaments that you can buy and place in your garden or areas you don't want your dog to go. These will only work if your dog doesn't like water. These work great because you aren't part of the equation. Instead when your dog ventures into an area you don't want them to be, their motion sets off the sensor which sprays them with water. If they don't like the water, it should startle them and discourage them from going in that area.
2) Prevention is really important, the more she digs, the more she learns that it's something fun and acceptable that she can do, and since some dogs are already natural diggers, this habit can be really hard to break. If the motion sensors won't work, then you'll either need to temporarily not allow her in the back yard unsupervised or bored, or build a taller fence that she can't jump.
3) It is possible to create an alternative digging area and encourage her to dig in the new location instead of the location she's digging in now. Not everyone likes to do this for obvious reasons....
I've attached a quick training tips sheet and a link to digging info sheet:
http://www.ddfl.org/behavior/digging-probs.pdf Please let me know if you have other questions, Jen
