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.
Think of it this way: how do you feel about fighting back physically against someone trying to hurt you? If you wouldn't punch someone, don't put a spell on them. If you're uncomfortable with either idea, focus on protective magick, for yourself, rather than something targeted on another person.
it is never a good idea to cause harm to anyone your best bet if you are 100 percent sure this person is harming you with magick you need to cast a protection spell around your self. there is some good karma spells here also this way you arent the one to be doing harm you are leaving it to karma..
Remember the ORIGINAL Law of Three: If someone cursed a witch, that witch would return the curse upon them threefold. If the original sender survived, they would know not to mess with said witch again. But if you want protection, pray to whatever deity you believe in.
Sorry Mesarthim13, but I just read through the Ardanes, AKA the original set laws of Wicca as set forth by Gardner. It contains the following entry on using harmful magick;
"So it is Ardane, that none shall use the Art in any way to do ill to any. how evermuch they have injured us. And for long we have obeyed this law. 'Harm none' and nowtimes, many believe we exist not. So it be Ardane that this law shall still continue to help us in our plight. ' No one, however great an injury or injustice they receive, may use the Art in any to do ill or harm any.' "
The little I played with spells and feel trying to hurt you. First part if you thought and said something about it, cancel it. The other way is to try some of the spells that break spells. Breaking and protection will depend on your strength. Both should help, do not think you get negative effects from breaking but I am novice, more advantance people will know better about that.