Force users to add Organic Groups content only to their group(s)
At admin/og/og (D6) you can uncheck the setting "Audience checkboxes". From the description "Show each group that the user is a member of as a checkbox in the "Audience" section. This enables the member to place her post into multiple groups. If unchecked, simplify the user interface by omitting the checkboxes and assuming user wants to post into the current group. This simplification only applies to new nodes, and not to edits of existing nodes. Group administrators always see checkboxes."