You could also mark all files in grid view and hit delete. Then answer "remove from catalog".
Unless you had autosaved previous developments to xmp.
Because then these settings will be read again in reimporting.
In that case you should use a copy of your images, untouched, to save over the xmp-polluted files.
I have some advice to get computer-literate in Lightroom terms, funny and very efficiently:
Get a copy of Victoria Bamptons book (electronically and/or in paper print) about the missing frequently asked questions:
Europe, Middle East and Africa