Individual entries can be hidden rather than deleted.
To hide an entry;-
Select the entry in the key (see Selection Tools).
On the main menu bar, select Edit > Hide Selection .
To show key entries that have previously been hidden;-
Select the key (see Selection Tools) or the key label.
On the main menu bar, select Edit > Show Hidden Items in Selection.
More information on hiding both key and chart elements can be found in Hiding and Showing Chart Elements.
Alternative sources of explanation: