http://wiki.mcbans.com/display/MCBans/Global+Ban+Rules
Just thought I'd post this info here. The highlights:
Global bans are to be used with the user commits a severe rule violation, as this significantly impacts their gameplay. Valid reasons for Global Bans are, but are not limited to:
Griefing, (Destroying items/blocks NOT owned by the user)
Causing harm to the server
Hacking (Exploits)
X-Ray Modifications (Texture packs or otherwise)
Racism/Homophobia/Sexism etc.
Nazi Symbols