Technically, you can by protecting a specific folder that holds all topics. But search, TOC and the like will work. So it's only a minimal solution and can be very confusing to readers. Some info here: Password Protecting a Folder
You can also completely protect an output using the method referenced above or by using the separate RoboHelp Server.
Wouldn't it be easier to create two outputs? One fully open minus the specific section and one with all content, but completely protected?