To intentionally remove someone from your circle of friends by avoiding contact, hiding or in extreme measures moving away. This will normally be necessary due to dull behaviour, fun sapping or heightened arrogance.
Dunbar's number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. The number is 150 and was first proposed by British anthropologist Robin Dunbar.
"Oh, I dunbarred him ages ago. He was a tosspot"