Ugh..this person is driving me nuts? will she go away?

ok i have a problem here. theres this 25 year old woman i know and she acts very immature and childlike. i heard shes very childish and she acts like she is mentally retarded. she works in a local grocery store that my mom and i shop in. for the past few months she has been calling and texting me. shes even talked to me online and is my friend on myspace. :/ everytime she texts or calls me, i dont answer. i answered her text yesturday saying my phone is broken (its not) and she wrote "sorry". then today my mom and i went shopping i tried so hard to avoid her, she saw me and said "ill text you later." i got so angry. then after shopping, my mom and i went to the bank, moments later i heard this voice behind me it was her she said "what are you doing?" OMG she wont leave me alone. how can i tell her to stay away from me? i want to her to stop calling/texting me.