Open Terminal, then ask
launchd to stop it:
$ launchctl unload ~/Library/LaunchAgents/ws.agile.1PasswordAgent.plist
That will immediately stop the 1Password agent, but won't prevent it from starting again on the next boot. If you want that, add
-w to the command:
$ launchctl unload -w ~/Library/LaunchAgents/ws.agile.1PasswordAgent.plist
To re-enable it without rebooting, substitute
unload in those commands.
/Library/LaunchAgents/ws.agile.1PasswordAgent.plist also exists — as it does on my system — you might have to repeat those commands for that file, too.
Regarding Lingon (recommended in another answer) beware that the current version has moved to the Mac App Store, and as a result of the MAS sandboxing policies, it can no longer modify system-level services. Since the 1Password agent seems to install
launchd entries at both the user and system level, it might work for this, and it might not.