It has been awhile since I did any WinHelp work but I think you should be able to do this via the DisableButton startup macro. Take a look at this macro under the Button category.
Thanks, Andrew. It appears to me that macro and its companions, CreateButton, EnableButton, etc., apply to Help buttons only, not tabs:
DisableButton WinHelp macro
button-id The identifier assigned to the button by the CreateButton macro. The button identifier must be enclosed in quotation marks.
Also, the RoboHelpExShowNavPane macro looked promising, but it hides the entire navigation pane, not an individual tab, and takes no parameters:
RoboHelpExShowNavPane WinHelp macro
This macro provides the functionality for the WinHelp 4 Explorer view – including the show/hide feature for the navigation pane.
RoboHelp automatically places this macro in the Startup Macros section (Project tab, Project folder Startup Macros folder) of your help project whenever you select this feature.