shared computer

i have never seen a shared computer on my finder sidebar on my home wifi till the other day when a friend had a macbook over and was using my wifi. they have left and no one else is using my wifi but it still shows a computer on there. I just upgraded my router firmware and have new passwords for both the router and the wifi and it still shows that a macbookpro-12characters is on my wifi. wenever i try to connect to it, it just says connection failed.. how do i remove this? i want bonjour computer still on my sidebar so i cant just uncheck it.
is there any way to delete something to remove this computer?

thanks for any suggestions/info