The vent can be blocked by removing the cover and stuffing a piece of insulation in. That will stop the airflow and will not be combustible. A better option is to apply a grill to the kick plate of the new cabinet. Prior to installation you can glue 1×3 or 1×4 (depends on height of kick plate) boards to the bottom of the cabinet to direct the air flow from the vent towards the grill. You will then get AC/heat in the kitchen as intended without any difficult modifications. Use a nice brass or wood diffuser on the kick-plate cut-out and it will look good and be functional.