You can revert an import from inside Nutshell by accessing Company settings > Data > Import. By scrolling to the bottom of this screen, you will be able to review all imports performed in Nutshell.
To undo the import and remove all of the people or companies that were created during that import:
Click the text that says "Completed" to the left of the import you are interested in reverting
Select the gray "revert this import" button.
If you are re-doing the import, you can select "Import with a new file" and import your data again using the same or a different CSV file, with the opportunity to change your field matching selections