imagent is launched via /System/Library/LaunchDaemons/com.apple.imagent.plist. I wouldn't recommend messing with that, however, because it's in the System folder. Instead, maybe try reinstalling Facetime (not sure if there's an easy way to do that in Lion, though Pacifist might work). Also, you can delete the cache and preferences, located in ~/Library/Caches and ~/Library/Preferences. Both are named com.apple.imagent (the preference file includes .plist as well, of course).
jW