This isn't documented on this delete procedure but the directions below are based on the Apple iPhoto help on using keywords combined with my delete instructions.
Add the Delete Keyword to your keyword list:
1. Choose iPhoto > Preferences.
2. Click the Keywords button at the top of the window.
3. Click Add.
4. Type your new Delete keyword (I use Delete but you can call it anything you want)
Tag your photos the Delete keyword
1. Select the photo or photos you want to delete.
2. Choose Photos > Get Info.
3. Click the Keywords button at the top of the Photo Info window.
4. Select the Delete keyword you want assigned to the photo. You can pick as many keywords as you want by the way.
Delete your photos with the Delete keyword
1. Select the Library icon
2. Click the Keyword button (shown in the lower left hand corner of iPhoto that looks like a key).
3. Click the Delete keyword.
4. Only the Delete photos show up. Just select them all and Delete.
5. To display all photos again after you've finished a search, click the Reset button.
Works really well. I got the idea with iTunes by tagging my music that I want to remove since it has the same issue.