I don't know the size of your yews (Taxus species), but the only sure way to protect them from deer is to surround them with hardware cloth or other types of fencing. You can find rolls of hardware cloth with 1/4- and 1/8-inch openings in the fencing department of your local hardware store. If your yews are the upright type and quite mature, you can fence the lower portion and then use one of the repellent sprays that are available. However, these sprays have to be reapplied frequently. Some people hang hunks of strong-smelling bar soap on tree limbs, although with limited success. As with anything, deer become used to that smell, too.