How do i get my boyfriend's ex to back off from our relationship? help me please?

God i am madly crazily stupidly inlove with this boy. i cry every night from just thinking about him, he is always on my mind that i have changed my lifestyle so that i could forever be faithful to him, i see myself getting married to him. am afraid of loosing him, i dont want to loose this boy, it would just kill me i swear but his ex has refused to leave him alone! she is constantly texting and calling him! what girl would text her ex at 4am telling him that shes looking for a house.?? he told me immediatly when he receieved the message from her and i asked him what he replied back saying but he told me not to worry about it. what do i do??? how can i get this girl to stay far away???? i dont want to loose my first and true love, the only man that has the key to my heart. what do i do? please advice me i beg you. am i been insecured?