How do I remove unwanted page formats from the system so they don't appear on New/Modify Page, etc.?
Page formats are stored as drawing files. Removing them from the configuration is accomplished by deleting them or moving them from the page formats path.
Use Windows Explorer to delete the files with file names matching the unwanted page formats, or move them to another folder.
Note: as-installed page formats can be found in the .zip file described in the Default Data folder contents article.