You have three choices, ignore their texts until the person gives up, be honest and tell them to stop or change your number.
I am not interested. Go away!! And if that doesnt work you can call the network and they can bar him from calling you.
Tell them<br />
"stop texting me, I don't like you anymore"
Tell them you cant talk to them right now because you are to trying to get your phone number changed, that somehow you picked up some crazy stalker. Then tell them, they can call tomorrow at your new number (Insert phone number of local police department).
block them. and if they use a private number and you get to know its them, ignore them completely