How can I disable/hide certain themes in multisite? I only want certain themes to be used for now.
You can disable certain themes in your WordPress network.
http://www.yournetwork.com/wp-admin/network/themes.php click on 'network disable' to hide the themes for the other websites.
You can enable a theme for a single site by searching for the site -> click on 'themes' -> click enable at the desired theme.
Hope this helps.