How do people that get banned form forums get back on?

Dont they get permanently locked out and their IP banned?

Why is it trolls can get banned and come straight back on to mock the moderators that...

Source: http://www.mmaforum.com/mma-forum-lounge/105983-how-do-people-get-banned-form-forums-get-back.html

Mike Weaver Randy Couture James Toney BJ Penn