Currently (it's on a todo list already, I think), there's no way to not log into Impulse automatically, but the workaround is to run the following to clear out the saved login data:
http://sd.stardock.com/curthendzell/fix_registrations.reg
Do not (I repeat, do not) run it while Impulse is still running, or you'll have to reinstall Impulse entirely. Impulse Now is not an issue, though.
If you run it after you're done with Impulse, though, it will kill any of the saved data, and it only takes a few seconds.
