Sorry, you received poor advice and poor quality soil from the nursery!
Yes, if you left the original rootball mostly intact, then undo the repotting by gently removing all of the soil you added. That should take care of the critters. Then, put the original rootball back into its original pot or one of similar size. If you need to add a small amount of fresh soil to fill in any gaps, use a mix of 5 parts peat moss and one part perlite. They both are pest-free.