How to clear shoping cart(allow to buy only 1 item per user) and user details

[electronic]

Hello,

i'm creating a website for registrations to conference - http://icolc.joomlatvs.lt/.
In my website users  have to fill billing information first and then they have  to select the product. No user registration required. Only one product in my store but it can have many variations because of custom fields.

Can you tell me how to allow users to buy only 1 item at a time. The problem is that user can add the same product multiple times to shopping cart (when user press back button before payment he can add another product to shopping cart).

And can you tell how to clear user contact information from home page after purchase is completed? All fields are still filled after the purchase is completed. My front page is edit user information window.


Regards
Arminas G.