How do I delete a CiviCRM instance?
The files are easy enough to get rid of - just delete .../wp-content/plugins/civicrm and best to clear ../wp-content/uploads/civicrm as well.

For the database, you could just leave the old version there or you can use phpMyAdmin to delete all the tables starting civicrm_ leaving all the ones starting wp_ there.

Mick