You have three choices, ignore their texts until the person gives up, be honest and tell them to stop or change your number.
We only do what works. If you never ever give a response of any kind, you wont feed this one way communication. Its best if you decide not to engage with it any more - at all. Then, you stop reading the messages, just delete them. You'll feel much freer, you'll have freed yrself. Very important not to make any exceptions to this rule, or you'll be hooked into the nonsense again. Be calm, be strong, be free. You can do it!
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