How to stop CI from continuing?

[eluser]louis w[/eluser]
Thank you for the suggestion, won't work in this instance thou. I was hoping not to effect the normal execution if possible. Let me go into more details.

I would like to have a conditional in my constructor which checks to see if the user is logged in. If not it would present a log in form and stop the page from displaying as normal (but not change the url). But if the user is logged in, not do anything.