Hi,
You can mark them as hidden in the admin; to delete them entirely from the system, you can log in to the database however you want (`psql fixmystreet`, or install some form of database admin interface) and delete the rows from the table or truncate the whole table.
Alternatively, you could export the bodies/categories using the bin/export-import-data script, recreate a fresh database, then import the bodies/categories back in.
ATB,
Matthew