We can state this as a misuse of the privileges given by the forum to the banned users. I think giving them a chance to login and change the profile details is a very stupid thing at first.
Banned account is warned with the message.
Sorry xxxxxx , you are banned from posting or sending personal messages on this forum.
Which implies they are banned from posting and sending PMs, but they are free to change there profile details. This mostly does not affect other members but there could be possible abuse as stated in the OP.
it might be a bug like what happened when merit system was implemented the banned users are also got merits and able to spend it.
It's not a bug, as you can see that the trust page of
btc-e.nz says "This user's email address was changed recently" which points he has personally changed it to spam @veleor (as per the thinking of scammer) as he has busted his account. It's just a way of him expressing his anger for this event