John Scalzi's recent post, Speech, Conversation, Debate, Engagement, Communication, is awesome. It's just a list of things that are true about speech, relating to asshats on the internet. Here's a section. You should go read the whole thing.
3. No one is obliged to have a conversation with you.
4. If they are having a conversation with you, they are not obliged to give you the conversation you wanted or expected to have.
5. If you challenge someone to a “debate,” they are not obliged to have a debate with you.
6. If they do not debate you, this does not mean you win. You can’t win a debate the other party has not agreed to.