There are several ways you can remove an element from view. Select an element or group by pressing its title then press the Del key:
Hide element will mark the element as hidden, but otherwise keep it in place. Users with edit or admin privileges will still see the element in the tree on the left greyed out with a strikethrough line, so that it can be selected and un-hidden. To unhide, from the tree press on the hidden element, go to the "Advanced" button in the toolbar, and select "Hide/unhide." You can also use the keyboard shortcut, Shift + D.
Delete element will delete the element from the project, but if the element is a group, its children will not be promoted a level. If those elements appear in other tabs by reference, they will still be there. Otherwise the children could become stranded elements, existing in the project but not actively referenced.
Delete element, promote its children will delete the element from the project, but if it is a group with child elements, they will be promoted up a level.
Delete element recursively will delete the element from the project, and if the element is a group it will delete its children, and their children, recursively. If those child elements appear in other tabs, they will be removed from there too.
Below the set of choices, a list of elements to be affected is presented for confirmation.
Editor only: It is also possible to make an element or tab visible only for users with edit or admin permissions, but hide it for users with view permissions. Press the edit icon and from the context menu choose "More properties...". Choose "Yes" next to editorOnly.
The tab will appear with a lock symbol in its edit icon: