Don't just try to deter the behaviour, find out what's causing it.
If it started lately, is there anything that changed in the garden or in the dogs routines?
What age and breeds are the dogs? Some dogs were bred for digging and will keep doing it. If this is the case, make a sandpit somewhere for them that they can dig in and hide interesting toys in the pit, then encourage them to dig there and reward / praise them when they do. Then you can start discouraging digging elsewhere.
If you have a dog like a terrier or Dachshunds which were bred to dig for and kill rodents, They could be hearing or smelling moles or other rodents. If this is the case, about the only thing you can do is to get used to it or get rid of the rodents.
Another common cause is that the dogs are bored or has too much exercise. Labradors for example will dig if they don't get enough exercise or mental stimulation. Obviously the solution is to give them more exercise or toys too keep them busy.