Validation control fails but still postaback occurs

Hi,
 
I have a page where user can create their account. I have several validation controls on the page.
When user clicks submit button the message box pops up with appropriate messges(as configured in validation summary). But once users clicks the OK button, then the form submits. Ideally it shouldn't do it?
 
I don't have any clue why it happens?
 
Can someone help?
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