Resetting the Pose should normally reset expressions (or set it to 'neutral'), at least in most cases. Do note of course that if you reset the Pose then whatever animation was ongoing, along with the character's very position in the Scene will also be reset and will nullify animations and possibly break the Scene if there's any motion involved. So even though it might be the easiest way to do it, it's probably the last one to try.
If it doesn't work then it's probably a Plugin set to the Atom that creates Expressions, and clearing / removing that Plugin could also do the trick.
If THAT still doesn't work then I'm not quite sure as I type this, other than maybe specific Morphs being scripted for the character in that particular scene and are 'forced' via the Female Morphs tab, if you were to click on "Active Only" would show which Morphs are being currently used by the character in the Scene. From there, you'd manually check which Morphs might be the cause (usually Morphs that have keywords along the lines of Smile, Expression, Anger, Sex, and more). And resetting those specific Morphs if you do find them would gradually revert everything back to neutral as you set each one to the 0 value (or 1.0, or whatever the default value is for that Morph).