Then, change a Viewport to one you want to delete, and use Snoop Current Selection (RevitLookup). You can drill down to find the ElementID of the Viewport Type Definition:
Ok, with this in hand:
- Change the Viewport back to one you want to "keep"
- Select by ID - use the ID you found above
- Press Delete on the keyboard (if any viewports still use this definition, Revit will prompt you that deleting this Viewport Type could cause problems etc).
- Rinse and repeat
Keep in mind that some Viewport Types are "special" / built-in, you will not be able to delete them all...