In the add to cart button behavior, set the quantity to 1 and uncheck the option for the quantity to be updatable. This will force the add to cart button to use the quantity of 1.
also, in the If Item In Cart section, set it to ignore or replace so the item cant be added a second time.
In your cart display manager on the cart page, remove the quantity text box.
another option is to create a calculation as:
[Quantity] = 1
this will force the quantity of each line item to 1.