Magento - Limit form submission per client

In the site I'm working on, there's a section where the client can rate the website and get a coupon.

In order to prevent them to vote multiple times, how could I limit their submissions to just one time?

I wouldn't like to use cookies because they could be erased, but rather I was thinking about the

Mage::getSingleton('customer/session')->getCustomer();

method, I could set a sort of variable to check if he voted already or not.

Any suggestion appreciated.