We too had this problem. they dug up and ate over 3000 tulips. You have to find where they are getting in and block off their access. In the end we had to put wire netting all round the garden perimeter. No sooner had we stopped one spot they found another. We are fortunate in that they cannot dig in as the boundary is full of bricks and boulders. They can and will climb fences. Ours is now chicken wire, but loose so that climbing it is not easy as it gives them no support. Also we have fastened strips of plastic to the fence as they are supposed to dislike the flapping. We were also told to put half full (or half empty) clear plastic water bottles against the fence as that is also supposed to discourage them. Something must work as we have not had any in the garden for a couple of years now. That could be because the main culprit had run in with a farm vehicle, sadly.
Best of luck!