Ajax ratings issue in CI architecture...

[eluser]Zeeshan Rasool[/eluser]
Hello,
I am using Ajax ratings script in a CI site but i am unable to stop a user to rate twice, i am calling a controller function thru Ajax and passing to it 2 param one post ID and second user ID but where will i put a check to stop it again? if user make a rating and refresh the page then he cant make rate again as i am checking from db if he is already did that . But if he first time make a rate and could't refresh page and again click on rating then i am unable to put a check in javascript . Please any idea ?
Thanks in very advance !