Restrict accounts below Member rank from posting in Meta. Someone with a Copper Membership can post as well since there can be people getting banned or hacked who would want to open a thread in Meta to get their problems resolved.
Accounts that are banned mostly don't deserve any consideration since they are rule breakers, and users that get hacked should be able to pay $10 or so for a Copper Membership if they really think their account is precious for them. So I don't think it would be a lot to ask for people in both of these situations.