How:
CNode::Misbehaving(int howmuch) is called when code detects that a peer is doing something out-of-the-ordinary. In this case, if a peer sends you a block that can't possibly be in the best chain, Misbehaving(100) is called.
When a Node accumulates a misbehavior score greater than or equal to "-banscore" (default 100), their connection is dropped and their IP address is banned from reconnecting for "-bantime" seconds (default 24 hours).
When are they un-banned:
After -bantime seconds. But the list of banned IP addresses is stored in memory, so if you shutdown and restart bitcoin everybody is un-banned.
There is no way to see the list of currently banned nodes, but you will see "Disconnected n.n.n.n for misbehavior" and "connection from n.n.n.n dropped (banned)" in the debug.log.