I wouldn't ban him.
If an 8-year-old girl came to you, told you that you're not her best friend anymore and now she's Susie's best friend, and that she's really been Susie's best friend all along -- even though she keeps coming back to your sandbox only to remind you of this -- that's not grounds for banning per se.
She may be a retarded, socially-stunted 8-year-old child. But that's not grounds for banning.