How can I make outlet wall plates mount flush after removing wood paneling?
Presumably the issue is that the outlet boxes protrude from the drywall.

Before doing anything else, turn off the power to the outlet.

Generally these boxes are typically nailed into the studs.  If this is the case, simply pry the nails out, reseat the boxes deeper in, and re-nail them into place.

If you can't reseat the boxes deeper (e.g. if something impinges on the rear of the boxes), there's a few options.  Replace the boxes with shallower ones.  Use a dremel or similar to cut off the protruding parts of the box.  Or add a "frame" around the box.