If you use Image Capture, it has an option to delete after import ... however as far as I know that only works at the time you import the images, not afterwards.
Since the photos & videos remain on your phone, try connecting your iPhone again, open Image Capture, select the photos & videos you want to remove, select "Delete after import" and try "re-importing" to a different folder than the one you originally imported them to.
If all is successful you will end up with a duplicate set of photos & videos on your Mac that you can, in turn, delete if you want to. And they should be gone from your iPhone.