I can't send texts to 'stop' on iphone

I keep getting texts from this pizza company about codes, and it says 'reply PXYZ to STOP' but i can't text to stop on my iphone?

You have to text the letters PYXZ to the number the texts are coming from. For example, if the number that the codes are coming from is 12345, then you send a text saying PXYZ to 12345.