Fixed quantity of items for a product

Version 1.5.1.3. I am trying to create a shop with a fixed quantity of products the customer has to buy.
Example: the customer can only buy 3 / 6 / 12 /24 /48 or 120 items of a product.
I have set the minimum to 3, and have created an option with the above fixed quantities.
The problem is that you can still put 5 items in the basket. How can I set it that only the required items are allowed?
Any help is welcome.
MAI

replace the inputbox with a combobox in your template.
G.