ANSWER - I need to be upfront with you that I am not a Wiccan witch and although I do have some knowledge of the Wiccan religion it is not a faith I personally practise. I can advise you on the general practise of Witchcraft but if you want the specific viewpoint of somebody who practises Wicca then it might be worth looking for a Wiccan witch's perspective. Certainly the whole concept of harming none doesn't factor in my own practise. Given the situation here I'd be inclined to hex the idiot.
Having said that I believe it is important for individuals to work within the parameters they feel comfortable within so we'll note the fact that you do believe in harm none and seek a solution that doesn't contravene it.
But firstly what mundane steps have you taken? This man comes onto your property on a regular basis and makes threats against your family. He also appears to disregard your daughter's disinterest in him. Have you spoken to the police about this? Have you told the man clearly and firmly that you and your family want nothing to do with him? I'd certainly suggest that you start keeping a diary and logging times and dates of each instance so you have evidence if his behaviour escalates. You say he is telling your aunt lies. Why is she talking to him? Often the best way to deal with an attention seeker like this is to cut off his audience. If you all agree as a family to have nothing more to do with him then chance are he will go cause his drama elsewhere.
From a Witchcraft perspective there are things you can do to cleanse your property of his influence and protect it. I would suggest that two of you walk the perimeters of your boundary together with incense. The first should use a cleansing incense (Wiccans would favour Sage for this) and the second should follow with an incense for protection. This could be an incense commonly associated with protection (Amber, Dragon's Blood...) but if I were you I would use a fragrance that has a strong meaning for those in the home - something that stamps your personality on your boundary. As you walk the boundary, focus on creating a mental barrier between the outside and your own land and visualise this barrier being penetrable only by those wishing you no harm.
For the home itself you can seal your doorways with a dilute solution of your own urine - this marks the territory as your own, stakes your claim on the land and asserts you as the dominant presence on your own land. You can also dig down into the earth, arrange the stones you form into the spelling of your family name and then cover the ground with earth.
I do have to point out that just because somebody says they are working nasty magic doesn't necessarily mean they either are or that they are capable of it. I judge witches by their results and if we look at his results the only outcome of his so called attacks on you is upsetting an elderly lady by telling her lies. Not exactly hellfire and brimstone is it? Until he manages to back up his claims with something more threatening than spinning his car in your driveway then I think it is pretty safe to assume he is somewhat exaggerating his own abilities.
Incidentally if you ever get bored of being nice, write to me again and I'll tell you how to hex the bastard!