This seems to have solved it for me, though I can't say how safe this is. I ran the following commands from Terminal:
launchctl unload -w /System/Library/LaunchAgents/com.apple.UserEventAgent-Aqua.plist
sudo unload -w /System/Library/LaunchAgents/com.apple.UserEventAgent-Aqua.plist
(I ran it the second time as root because the first invocation gave an error, about not having permission; the second one also gave a different error, about not having anything to unload. My guess is that the second command disabled UserEventAgent).
Anyway, UserEventAgent is now not running after reboot, and there isn't any noticeable problems with the system.
My
guess is that to enable this process again, I need to do:
sudo launchctl load -w /System/Library/LaunchAgents/com.apple.UserEventAgent-Aqua.plist
And again, as disclaimer, I have no idea if this is safe or not.
Message was edited by: casey.marshall (removed mistaken quotation of parent)