If you're talking about a place for discussion:
* Silence: Implies a lack of conversation or debate.
* Secrecy: Suggests a lack of open discussion or public involvement.
* Monologue: Focuses on one person speaking, rather than a shared conversation.
If you're talking about a platform for public participation:
* Exclusion: The opposite of being open and inclusive.
* Suppression: The act of silencing or preventing participation.
* Control: Suggests a lack of freedom or autonomy in decision-making.
If you're talking about a specific forum:
* Individual: Refers to one person instead of a group.
* Private: The opposite of a public platform.
To determine the best opposite, consider the context and the specific meaning of "forum" in your situation.