Hi Nahid,
Sorry about that!
What I should have said was that's currently not possible for you to do personally on CommCare HQ---it requires me to intervene.
It's not currently an operation that we expose directly on to users on CommCare HQ because it is such a large irreversible operation that we were concerned that users would do it accidentally and then want their data back. The easiest option is to leave your abandoned domain in place (it's not doing any harm).
However (and this applies to everyone) if you need to actually a domain and all its data, please send an email to me directly and i can take care of it for you.
Cheers,
Danny