In order to disable some items in the Context Menu of the HTML Editor, you should modify the file \sitecore\shell\Editor\Configuration\AutoConfigure\Full.config

Please have a look at the contextmenu section.

<contextmenu> <item name="Editing" value="False" /> <item name="Format" value="True" /> <item name="Insert" value="True" />

If you want to disable an item, you should set appropriate value to False.