Ah. So.. the PoE client is sending the MAC address to GGG via the PoE client. That is possible.
WIMM
Back around Feb-March 2013, they updated their account security stuff to try and reduce hacking attempts and access to accounts from different PCs/locations. Based on my logs, around the start of March, they added the client hash to the login packet as well as another hash of data, which I presumed was related to your MAC address. Here's the actual announcement, but of course it doesn't go into exact details:
The 0.10.1d account changes.
It's certainly possible they are now using that hashed info to associate accounts that are trying to bypass multi-client restrictions for the purposes of botting or RMTing (or just flat out botting). There's no way to know for sure, but I've had a recent report of someone who was getting banned over and over on one specific PC no matter what the IP was set to, yet his other PCs were fine. The MAC was not actually being changed, and there's enough people not getting banned/flagged to lead me to the conclusion they are most likely associating the accounts with the login info
somehow.
Anyways, it's always a good idea to change your MAC/IP after a ban. There's a lot of things they
could do, but aren't, so if you keep on getting banned over and over, you just have to assume they've found a way to tie your accounts together. That's not to say it could be from how you registered the account either, but it's assumed people understand that they need to register their account using the new IP they are planing on botting from.