It is very easy to delete your existing Lexicon library and start over.
On the Import page, you will see a checkbox Merge with current library. Disable that and when importing, Lexicon will delete all your playlists and tracks before starting the import. Effectively you are starting fresh that way.
Completely clean
If you want to start even cleaner (this also deletes your settings), then delete the Lexicon database file here:
Windows:
C:\Users\user\AppData\Roaming\Lexicon\main.db
macOS:
/Users/user/Library/Application Support/Lexicon/main.db
Exit Lexicon before deleting the database. Replace user with your username.
Videos
Check out our YouTube for videos about Lexicon.