Question
Hi there, Is there any 'safe' substance that can make squirrels not want to Dig holes in your lawn ?  
Thanks & regards

Answer
The short answer is no.

All the options I can suggest to put a stop to it you won't like. They include,

1. placing 1/4" hardware cloth over lawn.
2. cut down all the mast producing trees.
3. paving the lawn with asphalt or gravel.
4. trapping down the squirrel population. visit http://icwdm.org

You can try all kinds of repellents, including those silly ultrasonic devices.  They do have placebo value in that they make people feel like something is being done. But like a sugar pill they won't work and can be pricy.

Sorry but there is no magic in this business.