EDIT:
I just saw this suggestion:
"Can you make option for user to transfer btc to another user ? We can help promote this site and do many giveaway by that"
I would strongly recommend you from refraining to add this option.
Seeing that you already have a problem with multi-accounts.
This will only provide those who want to gain btc like that to try even harder, and they will always succeed.
If I could vote, I would say no to this.
Yup that is why I said I would consider it. However, it is not necessarily a bad suggestion. There might be a way.
And I will keep you in mind for alpha testing.
You can make something like end 50 or 100 gold to your ref, and that would be one time only fixed amount.