How to seal of a hole in concrete (blocks) that mice are getting through into the garage?

We have had a slight mice problem. I hear something scratching in the garage, I thought it was a worker outside. The today I saw a mouse scurry into that hole. The bottom half of the inside of our garage is cement cinderblocks. My husband is in Iraq or else I'd have him do it. How do I close the small opening in the top where the concrete meets the drywall? (just in one area, not the whole perimeter)