A ban is, generally, any decree that prohibits something.
Bans are formed for the prohibition of activities within a certain political territory. Some see this as a negative act (equating it to a form of censorship or discrimination) and others see it as maintaining the "status quo". Bans in commerce are referred to as embargos.

Banned persons under Apartheid
In internet forums and MMORPGs, the host sometimes has the power to ban players who do not follow the rules.
Banning (sometimes called 'blocking') is commonplace in internet culture. The banhammer is a metaphorical tool used in many of these games.