The Entries[Disable] command prevents an entry from appearing in the context menu until re-enabled with Entries[Enable].

The parameter action specifies the action or actions to disable. If action is a string, any entry in the top-level context menu with this name will be disabled.  If action is a list of strings, this is interpreted as a hierarchical sequence of submenus starting from the top-level menu.  Any menu entries whose submenu entry sequence begins with action will be disabled.

To remove all references to a given context menu entry from the menu, use Entries[Remove].

with(ContextMenu[CurrentContext]):

Disable the top-level menu entry that negates a boolean value.

Entries[Disable]("Negate");

Disable the submenu entry that sorts a list in ascending order.

Entries[Disable](["Sorts", "Ascending Order"]);