Blocking someone on a macbook

Hello! I am trying to block someone on my macbook pro on iMessages. I have them blocked on my iPhone and iPad. All these devices are connected to the same iCloud account and the person I am trying to block is iMessaging me using their cellphone number, not an email. When I click on the "buddies" option after opening the conversation, the button to click "Block person" is grayed out. I cannot find a way to get this person blocked on my macbook and I need them blocked on all my devices. What do I do?