Dont like all this attention from guys?

i tend to get a lot of attention from guys, im only 18 and im modest and shy, i dont dress like a skank and i dont really wear makeup, yet i always have guys hitting on me and checking me out. its not just guys my own age either, like some could be in their 30 and 40, it creeps me out and i really dont like it. i've told them to stop, that i have a boyfriend, even that im a lol but nothing works, im tired of this, i mean i dont even like going out anymore, or going out alone. what should i do? and why do you think this is happening to me? i mean people tell me im pretty and beautiful but i never really put much thought into it, i dont really spend much time or thought on my appearance. and i dont have a bad rep as "easy" or anything like that either.