Is there a setting to ensure a user can't reference a hidden sheet?

MS Office is meant to share data, not for securing it. No matter what you do that information can be gotten to by someone with a little time/desire and Google. Your best bet is to export out just the summary sheet with values in place of formulas into its own file (either Excel format or an alternative like as a PDF).

There is a "very hidden" option, in the developer tab you select properties then at the bottom across from visible is -1 - slSheetVisible and in clicking the dropdown you will see 2 - xlSheetVeryHidden
The very hidden sheets will NOT show when right clicking on a tab for "unhide".

But caveats, as Zer0Cool noted it can be unhidden by someone with a good excel skill set. If for example you use formulas that refer to the very hidden sheets then sheet names will appear in them. TO UNHIDE the very hidden sheets you need VBA.

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Where "sheet2" is the name of the very hidden sheet.
If you use this you better note the names of the sheets because if you hide them you need the names exactly to unhide them.

Or someone could just Google "unhiding all sheets" and find a snippet of code to loop the sheets collection and unhide every sheet without knowing their names.

The only way to keep information from someone in Excel is to not include it in the file, its that simple unfortunately. All of the "protections" offered in Excel are meant to prevent accidents, not secure a file.

@Zer0Cool, if I wasn't clear enough I was trying to say you were right.

@Sambo kid I'm with ya, I just wanted to make it clear to the OP that it wouldn't take much to overcome such an effort such as very hidden. As you basically stated it'd likely be more of an issue for the OP than for someone trying to get the info.