How can I restrict a particular user from participating in a discussion forum?

This option is not available within the forums channel included within the Academus Portal (outside of the Acadmus Collaborative Groupware).  Access to the channel is managed at the group level, so all members of a particular group will be granted equal privileges.   

However, within a course offering in the Academus Collaborative Groupware instructors can restrict participants from the Discussion Forum channel on an individual user basis using the permissions capabilities within the Roster channel.