Magento – How to disable quantity edit in frontend for grouped products

Some Brief

I want to sell products like tea, coffee, juice etc they are in small packets each packet cost is around $3.73. For this i will make it simple product in magento. So when customer will purchase it. Its quantity will be deducted.

BUT

I also have 3 subscription plans, before becoming our customer he/she will compulsory have to buy subscription plan first, and it is one time only, then later customer can purchase individual products according to his needs in terms of quantity. But on first transaction they have to buy subscription plan only. Now, in that subscription plans i am selling the same simple products in multiple quantity that means in first plan coffee 20 packates and in second plan coffee 30 packages and so on.

Now, lets come to problem according to magento's flow.

If i enter all the product as simple products in magento admin, this will be okay for individual packets selling, but how will i manage them in subscription plans because when someone will purchase subscription plans we have to deduct 20 quantity from the same simple product

What i did

I have tried adding it as grouped product and i think it will serve my purpose, but in groped products also in Associated Products section there is column of putting quantity that is perfect for me. But that quantity is editable in frontend. Is there any possibility to disable it so that customer will buy the quantity what i want him to buy he should not be able to edit that quantity.

Any help would be greatly appreciated.