This was such a cool thing I've needed to do for a while on my Plone sites. Thanks to sfulmer for this info!
To hide tabs:
From the Plone Control Panel, go the the Zope Management Interface.
Click portal_properties, then navtree_properties
In the field 'IdsNotToList', enter the ids of the folders for which you do not want to generate tabs (one item per line). The ids of the folders I removed are: Events, Users and News are: events, Members and news, respectively.
Click 'save changes'
sfulmer also adds you can:
turn off automatic tab generation altogether and define tabs in portal_actions in the ZMI. To set a default page for a folder, use the 'display' content menu, click 'change content item as default view'