Try to look at this somewhat objectively. They have no idea if you're telling the truth, and you have no proof. Someone could just keep picking friends, opening up accounts for them, and do this over and over again.
If these people signed up at your IP address, and then played at other IP address (implying they left and played on their own accounts) you'd have more of case to make.
The books could do a much better job explaining these things to people when they take these bonuses, but they don't. Their stance is "if it's in our rules, we don't have to say anything".
Closing all accounts and nullifying all monies won from bonuses sounds pretty standard. Any balance in the account before the bonus needs to be paid.