You can use the following to completely seal off the top of the concrete block:
1) Use crumpled up newspaper, and stuff newspaper in the top of the concrete block core holes to take up space, then use mortar / portland cement to seal off the top of the block.
2) Cut XPS foam board to fit in the holes of the concrete block in order to take up space, then fill the remaining gaps with Great Stuff Big Gap Filler Foam to seal the top.
3) Stuff the top of the concrete block with Xcluder rodent control fill fabric, or Stainless Steel Wool, then fill the remain hole with Great Stuff Big Gap Filler Foam
4) If you're NOT worried about moisture / mold / mildew, you can stuff the top of the block holes with Fiberglass insulation, then fill the remaining with Great Stuff Foam.
To finish, you'll want to cap the top of the concrete block with Sill Sealer Foam Gasket and a Pressure treated piece of wood. Use Loctite PL 300 Foamboard Adhesive to attach the Sill Sealer Foam to the Wood and Concrete Block. You can also use concrete screws to hold the wood down to the concrete block. To make pilot holes in the concrete block, use a Hammer Drill.