The Blizzard End User License Agreement explains what you may and may not do with a Blizzard account. This article highlights a few passages from the EULA that are most relevant to this policy.
How to ignore another player in Diablo II (2000)
Blizzard's stance on inappropriate behavior in Blizzard games.
General information about the capabilities of Parental Controls.
Information on tournament formats for Blizzard products