Page Refresh click button!!!

Hi
I got a form with a usercontrol (Login Control) that I created on it.

The control got a Login / Logout button on it, which, when clicked, call a function to log the user in or out.

I could not understand why the user get logged out when I refresh the page (pressing F5).

After some debugging, I realized then when the page are refreshed, the user control's login button click event get fired.

Is there any way I can prevent this?