you would need to use eCart to modify the store for this.
you would need to create a calculation in the power store eCart object using the ID of the item.
the calculation would be:
abs([ID] == '22')?[Quantity]=1:''
this is an example using item ID 22. If that item is in the cart, it will force the quantity to 1 even if the shopper tries to set it to another value.