They'll just keep hacking ancient accounts that have megas like they do now. It won't remove the megas from those accounts.
How about this: if an account was inactive for more than half a year, then /y should be simply blocked. But the user would get an email with a confirmation link: clicking on it would confirm that it's her indeed, and would re-enable her right to world-shouting.
By the way, if breaking into inactive accounts is still such a problem, why a similar check (email with an unique link) is not done for logging in? If an account is inactive for e.g. half a year, first logging in could just show a message "Account blocked due to long inactivity period, please click on the link in your email to unblock."? It's just a one-time tiny annoyence, if done automatically based on a unique link would mean no extra work for support, and - would completely solve the breaking-in problem once for all, wouldn't it?

(And if anyone has lost an access to her email account, she can still contact the support and get the account back.)
Just to make it clear: there were two ideas mentioned above:
1. make second-hand dias (i.e. dias bought off AH) bound/marked/not-resellable;
2. limit who can world-shout;
In other threads (
e.g. this one) there was discussion about limiting who could use dias AH trade, which would be yet another idea (and - the same rules could actually be used for /y !

). Anyway, we don't need both (or all three), either one of these two would suffice, right?
1. If dias are bound they could not be a way to earn gold easily, so goldsellers would simply have not that much gold to sell - it's very close to the current situation: /y is not limited and still we don't get flooded with world spam.
2. If world-shout is limited (without binding dias), the goldsellers wouldn't have an easy way to get clients, so even if they could in principle earn gold on trading dias on AH they will probably just go to a different game...
GF/RW could ofc go for both, but it's double work, and both changes besides the goldsellers would limit also "normal" players so if we can avoid one of the two I'd rather not have both if it was my choice. Imho, restricting world-shouts is less harmful to real players of the two. Just my opinion, but if there was a solution for /y I would really rather _not_ have the dias bought from AH bound.
Having said that, any trade with as strict restrictions as you wish would be better than the no-trade situation we have now... So, sure, go for both bound second-hand dias (or limiting who can trade) and restricting the shouts if that's the only way to give us any dias trade back...
But ... we still didn't hear from GF what *they* see as the key problem blocking re-enabling dias trade. If, for example, the only thing they cared about was dias duping then a big part of the above discussion may be quite irrelevant from GF's point of view, I'm afraid...