Re: One solution...
No need for that really. A report button would be enough. I mean a person with a bank account is highly identifiable - you cant open a bank without personal ID, addresses, etc. So the sender of the message is clearly identifiable to the bank. If something is reported, someone at the bank identifies if it breaks the law on harassment and then passes the details on to the police to sort out.
If the senders try to get around this by using some sort of intermediary service. A bank threatening to no longer accept said intermediary service's transactions will very quickly see that service cleaning up its own house, and stopping the harassment at that point.
This quite frankly should not be a problem the banks cant solve very quickly.