Since your site is fluid width, it might be that simply reducing the width of the sidebar will make your users happier. That will require some tricky CSS though, since only newer browsers will support setting a fixed width on the sidebar and leaving the article content only as fluid. And the Javascript would have to be tweaked to deal with this. I'll see what I can do for the next release.
But yes, in the Admin Panel, under Content > Sidebar Blocks, you should see a "Modify Permissions" link. Permissions are typically positive only (e.g. setting the "nowikisidebar" group to all no would have no effect if the user is a "Registered User" and the "Registered User" group has all set to yes), so it would be easier to make 1 usergroup that CAN see the sidebar, make everyone a member of it, and make it possible for users to join/leave the group.
Another thing we can consider doing in an upgrade is making a user preference for whether they want to see the sidebar, as well as saving the collapse-state between page loads.
Otherwise the only option is to remove the sidebar for everyone, by disabling the sidebar (it's in one of the setting groups -- either Aesthetics or Miscellaneous, IIRC).