If you are truly looking to bring yourself protection from all things, carry rune stones or sprinkle some salt by your windows, doors, and entrances into your home.
You really need to let this person know that you have done something to ward them off because it is part what you put into a protection spell/charm/etc. and part what they perceive having been done. At least that is what my mentor taught me.
In my experience what really scares a witch is Bigotry, Ignorance, Hatred, Mis-understanding and Mis-representation. However I think what you were asking was how to scare a witch away...
First be sure there is a witch. Not everyone "looking like a witch" is one, and more importantly, not everyone "claiming to be a witch" really is... there are people out there who call themselves witches for the "cool" factor, or to impress or scare others, that is not a witch.
Second be sure there really is an ill-wish against you. If a real witch has ill-wished you, and that would be a rare thing as witches believe that what they send out will return to them three fold, then you will need to take action and the above suggestion is a good one, but does open you up to "doing witchcraft" yourself. If you are not Pagan and do not feel comfortable doing a spell yourself, then go to your own church and talk to someone there about a blessing. Do not run in claiming you are being attacked by a witch, this is not Salem... Also, check your own beliefs, a witch can not harm you unless you believe the witch can harm you.
If however you are just concerned because there is someone out there who is or claims to be a witch and you are uncomfortable with their attentions, you should take the initiative and speak to them directly... Witchcraft is not contagious, it does not rub off and simply sorting out the "difficulty" in a quiet, civilized and adult manner is usually the best path of action.
I'm a witch myself, and I can confirm that intolerant and bigoted people really do make witches flee; however, I hope that is not the attitude you will choose.
I wonder why you want to scare a witch away. Most witches, including those who follow the religion of Wicca, do not seek to hurt people. They have an ethical rule called the Wiccan Rede: "An it harm none, do what you will." This rule forbids the use of witchcraft to hex or manipulate others, except in cases of self-defense. Thus it is very unlikely that a witch will harm you if you don't harm him/her in the first place.
If you are convinced that a person is a witch and wishes you harm, you should start by talking to them. Maybe there was just a misunderstanding. In fact, maybe that person is not even a witch at all (wearing a pentacle or having a goth look does NOT make anyone a witch; most real witches dress casually and go unnoticed).
Should you be threatened by a real witch who does not uphold the Wiccan Rede, you can indeed sprinkle salt by your doors and windows to prevent bad energies from entering your home. You may also carry a spiritual symbol that is positive and meaningful to you (like a cross if you are Christian or David's Star if your are Jewish) to ward off harmful spells. However, those very basic protections may not completely block evil magick, and you may need to either ask good witches for help, or do some witchcraft yourself (like protective spells or calling on your deities, guardian angels or ancestors for protection).
ONE QUICK THING
One quick not on the things above. If the person is wishing ill intent then they are not a true witch for they do not follow the religion. They may still practice magic but does not make them a witch.
Jadeacres