The only sure way to protect your plants them from foraging eer is to surround them with hardware cloth or other types of fencing. You can find rolls of hardware cloth with 1- and 2-inch openings in the fencing department of your local hardware store. One-inch fencing will also keep out rabbits. If your yews are the upright type and quite mature, you can fence the lower 5 feet. Just be sure the fence is less than 6 feet in diameter to reduce the likelihood that a deer will jump inside. Also ensure that the fence is several inches away from the plant so deer can’t browse on branches too close to the fence.
Repellents may be another option depending on the amount of deer and food in your area. If the deer density is low and there is plenty of alternative food available, repellents may be sufficient to convince the deer to forage elsewhere. Follow the label carefully and reapply the product as directed. The online resources contain additional information.