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.
"stop texting me, I don't like you anymore"