The paid add-on Conversation Essentials allows the conversation starter to 'kick' (force leave) a participant who is not a moderator:
Option to allow certain user groups to kick a user out of their conversation via a link in the participant's mini avatar. A kicked user may be re-invited back into the conversation at any time. Please note that those users who leave the conversation on their own accord cannot be invited back as per default XenForo behaviour. Administrators cannot be kicked out of conversations, while Moderators can only be kicked out by Administrators.