It's a job for HR.
As this sounds like a widespread practice, not limited to one or two people, I'd suggest it's a job for HR to alert people and create and communicate a policy to the staff in general.
So I'd suggest discretely contacting HR and asking of they could e.g. email all staff (which would be anonymous from your point of view) requesting them to respect colleagues around them by not having discussions near cubical areas after meetings. Encourage staff to talk quietly in their own offices and cubicles and not loiter. Perhaps a more general "keep the noise down" policy.
This is only common sense, after all.
They might also post a notice near meeting rooms to reinforce the idea.
It probably won't completely eliminate the issue, but it may cut down the worst of it.
If you're in an open plan office it cuts both ways. People do have a right to expect to exchange reasonable off-the-cuff remarks (you don't want to create a repressive atmosphere), but they also need to respect those working around them by not loitering.
A somewhat more cynical way of looking at it is that you have an interesting opportunity to listen in on conversations and perhaps find out useful things you would not otherwise.