How can I rid my yard of moles?

We have a field in the back of our house and we have a fence but somehow they tunnel under the fence, even though I have a 6" paver buried under the bottom of the fence. I have been battling them for about 2 years now. I sprayed the yard with castor oil and spread castor oil granules, but this didn't seem to work and I had to resort to killing them, either with a shovel (4) when I saw them tunnel in a shallow tunnel area, or with a mole trap in a straight line burrow (2 bit the dust this way).

Now I have this one solitary mole left that managed to find his way into my backyard after I thought I had rid myself of their presence, that has managed to elude me. Most of it's tunnels are deeper than those that I have experienced in the past and whenever he comes up for air he either leaves a hill of dirt or a hole in my bermuda grass. How do I get rid of this last one that I can't seem to locate. The now extinct 6 moles managed to destroy part of my lawn, and don't want this one or any of this friends he invites over to do that again, or worse yet, undermine the foundation of my house.