Tunnelblick is designed to keep an icon that displays the status of VPNs in the menu bar. Hiding the icon would mean that users wouldn't know the status of the VPN, which I think is a Really Bad Idea.
Did you mean to ask "how can I keep Tunnelblick from launching each time I log in?" if so, the answer is to quit Tunnelblick before you log out, shut down, or restart your computer. If it isn't running when you log out, it won't run when you log back in (unless a VPN is active, in which case it will launch and show that VPN's status).