You can remove a participant in two areas
From Within An Active Message
Step 1 - When in your active message, click 'View Participants'.
Step 2 - Your selected participant(s) will be listed. Click 'Remove' beside the person you wish to remove from the active message.
The participant will no longer have access to or be able to participate in this message.
From the 'Add Participant' Screen
On the 'Add Participants' screen, simply uncheck the person you wish to remove.
If you wish to archive a 'guest' participant, see the article - Manage Guests