There is not a direct way to restrict current members from accessing the forum, but there is a work around. You would have to create a member plan that is the same exact one as the member is in currently, but would deny access to the discussion forum. If you do get a disruptive member, you could move them over to this member plan:
A. Create a New Plan for this member
1. Under 'Master Settings' choose 'Edit Member Plans'
2. Choose 'New - Add New Member Plan' from the drop down menu
3. Copy the same information from the member's existing plan into this new plan.
4. In the 'Forum Access' field select 'Allow Access to NO Forums'
B. To move the disruptive member to a new member plan:
1. Choose 'Edit Member Details'
2. Choose the member.
3. In the 'Member Plan' field, choose the new member plan.