Add the number to the blocked number list on your phone which most phones have built in these days.
If yours doesn't, look for an app in your app store which will do it, such as AVG or Avast etc.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)