Let's all get on the same page.

Set the expectations when you send out the invitations, and again at the start of the session by clearly stating the Rules and Process.

You may also want to post up the rules in the room as a constant reminder.

Many people are not used to thinking freely and playing with ideas. They need explicit permission to play, to remove (or at least reduce) the tendency to analyse and criticise any new idea.

The aim of a brainstorming sessions is not to determine a correct answer, but rather to generate large quantities of possibilities from which the correct answer may be distilled (in the subsequent phases beyond brainstorming.)