With community bans, they try and sever connections with other players. However the system is not perfect, since you can still receive tubmails, talk to people via INCIT subs, klan titles, BP transfers, klan invites, etc. (Klan title chats work best when both players are officers in said klan)
They only block the chat itself. Offsite IRC is another excellent way to get around the rules. So a community ban doesn’t really hinder you from doing anything. If you need a post/thread made, you could ask an e-friend to make it for you.
With full bans, access to the site is completely denied, so you’re restricted solely to offsite IRC.
With permabans, you get “permanently” denied login access (unless you are/were a cool kid, in which case such bans last for a month)