How to disable “Ship to a different address”

Hi.

I have problem with shiping address. I have on my website (checkout) 2 shipping options:

1. Billing address
2. Ship to a different address

Both are mandatory. I used have a box where my customers could select “Ship to a different address” but now they must manually fill in 2 addressess (although in 99% cases) they are both the same addresses. I tried to select 3 options in shipinng settings:

• Default to customer shipping address
•  Default to customer billing address
•  Force shipping to the customer billing address

But the result is always the same. Both addressess are ALWAYS mandatory. How to make just one address mandatory and another one optional?