There is no single answer to this. Different practitioners from different traditions and religions will come to different ethical conclusions. I feel that it is not only allowable, but demanded. You are a special and sacred being worthy of fighting for. I do advise using as little magickal violence as possible - try simply blocking their attacks first - but if your options end up being "be attacked" or "fight back" I say you cast something right back at them.
Amulets and Talismans are good ingredients for protection. Amulets are specifically and especially used for protections, this is specifically what they serve. It could be a ring, gem, plant, or any other item (sometimes even a word or phrase) but the most well known is a pendant. When making one, you cleanse the item and charge it to protect you from harm. A talisman is an object that is charged with your energies upon any specific purpose, whatever you will, designed for a particular purpose. It could be charged with your own energy, that of the earth, moon, another practitioner, even a powerful entity. Make sure that the way you do this suits the purpose you are going to set it with.
I would like you to check out the full article of protection magick written by a member under the username moondragon, it's splendid !