Phone harassment from agent. Any solutions?

i keep getting "new girls" sms from an agency that i haven't visited before, every week i would recieve sms from them telling me about new girls. i messaged back telling them to not message me again since its my personal phone, i even tried calling them but no one picks up. i know it is not a prank since i searched that number and alot of backpages ad pops up. anyone know how to get rid of it? help a newbie out