To delete a plugin preset you've created from the Waves preset menu, follow these steps:
Option 1: Delete the preset file from the User Preset folder.
PLEASE NOTE: Deleting a user preset is an irreversible action: you cannot undo it. Factory Presets cannot be deleted.
In V12, an additional preset browser has been added for fast preset searching, via the ‘Load’ menu. Instead of browsing manually through preset menus, you can now search presets by text, filter them by name, delete user presets and quickly audition your results. To learn more, please refer to the Waves System Toolbar user manual.