When someone is banned, they simply make an alt to get by it.
So:
If you do use an alt to get by a ban, your alt is banned and your main's ban is extended.
Likewise: breaking the rules on your alt gets your main banned, as well.
If they got banned, It was most likley for a good reason.
But british law states that if you do something that was later made illegal, you can't be punished, so I cant see why we should ban existing alts of perma'd users.