How can I block a phone number?

How can i block a girls number from my boyfriends phone without him knowing?

• okay, so. there's this girl I really hate. she's tried breaking me and my boyfriend up and she's lied to both of us before. when me and my boyfriend fight, she'll always text him. once I found a message on his Facebook where she said she liked him. recently me and my boyfriend got in a huge fight and "broke up" for a couple of days. but we made up, and are back together. the night we got back together I was over at his house. I asked him if him an the girl I dislike have talked at all. he said "when we were broken up". there's more to the story, but let's just say, she's a liar. and I don't think he tells me everytime they talk. And im worried. can I block her number off his phone? So she can't text him or anything?

• Answer:

  Yes you can but he'll figure it out somehow (and it will happen) and you'll lose his trust