As a moderator you have the ability to restrict or "lock" a viewer's ability to access media and other components of the BigBlueButton interface.
The lock settings are located under the user list. Select the cog wheel icon and choose "Lock Viewers.." from the drop down menu.
Next, select the specific feature you would like to restrict from viewers, such as microphone. Be sure to apply the changes once finished.
It is important to note that this particular setting does not apply to other moderators.
Once applied, the users list will show that the viewers have been locked. To unlock an individual user hover to the right of their name in the users list and select the lock icon.