Is there a way to unlink the previous BTC address? I have a new BTC address and I linked it, now I have 2 BTC addresses, can I remove the old one which has zero balance?
No, you can't unlink; however, you can get the transition bot to 'ignore' it if you create another transaction by trying to link to another byteball account.
I think you may have worried that you can't link multiple addresses? You can.