What Can I do About an Evil family member?

Its and in law and I am keeping very low tech, as the person in question is very snoopy and searches for me over the internet all the time they even quit their job to stay at home and constantly search for me, what I am to do this person emails me constantly and even though I continue to try to ignore it, this person keeps it up as if its were some game, I have dealt with this person for several years now, this person in no blood, other then in law .... I know some in laws don't get along but I get along with all mine but this person , this person turned this persons partner againest me, when we used to get along out of friendship and nwo we hate each other, on top of all this this person emails me threating harsh and very disturbing emails, as well as calls non stop over and over my family on my husbands side believes this person to be pyscho and needs help, This person would come over all hours over the night , and this person does these things just to see a reaction from me,