Sure you can undo the changes. There are a few possibilities.
The basic concept is that Lightroom (and other Adobe Camera Raw in Photoshop or Elements) does Lossless edits. I am not saying Photoshop or Elements does lossless edits, I am saying that Adobe Camera Raw in those programs do... This means that your original image (JPG or raw) is never modified, the original is pristinely preserved. ACR and Lightroom Never modifies the original image, it merely saves the list of your edits. You only see the edits because you are viewing in Lightroom, which applies the edits to your view every time you see it in Lightroom.
In the case of JPG, if you simply look at your original file with another editor or viewer, it will not know how to apply Lightrooms techniques, it does not know how to find or apply those edits, so you will only access the original image. If you want another program to see your edits, then Lightroom MUST output a new JPG version with the edits, which any other program can see then (and which cannot be removed from that second version then - but the original is still unmodified). So assuming you are discussing the original image file (instead of an new JPG outputted from Lightroom), just use another image viewer or editor to access the original, less edits.
Or, if your edits are saved in sidecar XMP files, you can simple remove or delete those sidecar XMP files, and then even Lightroom cannot apply those edits any more. The edits are gone, and all that remains is the unedited original file (lossless edits).
Or (even if edits are saved internally instead of sidecar), the image is still unedited (it is always unedited), it only contains the list of edits somewhere. So just have Lightroom restore the edit list to be no edits, an empty edit list. However, depending on what your Lightroom Default settings are, this would simply be applying Defaults to remove previous settings (but you still get whatever the default have been set up).