[DNSB]

The default settings allow you to prevent switching between portrait and landscape but turning rotation off entirely is not part of the default settings ( Settings => Reading settings).

NickelMenu does have an nickel_orientation setting which might override the default, i.e. setting it to portrait would prevent switching orientation but I have never tested that.

nickel_orientation - one of:
                        portrait           (4.13.12638+)
                        landscape          (4.13.12638+)
                        inverted_portrait  (4.13.12638+)
                        inverted_landscape (4.13.12638+)
                        invert             (4.13.12638+) - Toggles between inverted/non-inverted (preserves side) (this will not work if used in a chain with swap)
                        swap               (4.13.12638+) - Toggles between portrait/landscape (preserves inversion) (this will not work if used in a chain with invert)

So something like the following might work for you:

menu_item : main    : Portrait        : nickel_orientation : portrait