Sorry for the harsh review; I was feeling frustrated after banging my head against the wall over this problem.
After I deactivated NextGEN, there was no option to Delete. I even made sure all galleries and gallery images were deleted beforehand.
With all the other gallery plugins I was evaluating, I was able to delete them after deactivating.
I ended up deleting the ngg tables manually in MySQL, as well as the ngg directories in my file system (there were two that I found). I no longer see any notices or any other sign of the NextGEN.
I thank for you responding to my review.
