Permanent Ban

A permanent ban (aka perma ban) is 1 of the 3 types of bans you can receive on Miiverse. A permanent ban will usually happen when you get temporarily banned 4 to 5 times. However, this is not always true as some rarer possibilities have achieved a permanent ban. Permanent bans have been rumored to last only six months, but this turned out to be false. Typically, a permanent ban only permanently restricts one NNID instead of all. A console ban on the other hand restricts the console. Users that are perma banned are labelled as "perma'd".

The difference between a permanent ban and Console Ban
A permanent ban only permanently restricts a single NNID. If you keep receiving permanent bans, you're likely to receive a console ban. Permanent bans are quite common, and surprisingly, many users may have at least one permanent banned account. A console ban, as the name suggests, is much worse. If you violate the Miiverse Code of Conduct too much, you will most likely be console banned, and you will no longer have access to Miiverse on that console.

The Update of 6/24/15
After the update that restricts a user from all accounts after getting banned on one, many users assumed that getting a permanent ban would cause all accounts to be perma banned as well. This rumor turned out to be false, receiving a permanent ban will only cause other accounts to get a 2-week temporary ban.

6-Month Rumor
There is a rumor that permabans last 6 months, but this is FALSE. They are forever, until the end of the world.

Trivia

 * For Wii U owners, this isn't a big deal. However, 3DS owners will have to delete their permanently banned NNID to make a new NNID to use Miiverse. If you have stuff downloaded from the eShop that you got while you had your permanent banned NNID, you have to format then make an alternative account then format again with your original NNID but you will lose save files, you will have to re-download games, you cannot draw on the alternative account unless you don't reformat back to your original NNID and you're not able to post about games you own on the other NNID.
 * The amount of permanent banned NNIDs to get a console ban depends by admin discretion, but the most common one is 5.
 * It is in fact possible to get a Console Ban Warning without a permanently banned account.