I have found the ultimate evil solution! When they do not allow you to delete it, then just create a folder/document of the same name, and move it to the directory where the offending folder/document is located. The unwanted item will be deleted when it is "replaced" and then you are free to delete the file you have created.
This works extremely well, I actually deleted my system folder doing this and it didn't even complain!
[Editor's note: I haven't verified this myself against a non-deletable object, but clearly, in general replacing one file with another when copying works. If you attempt to use this to delete a stubborn item, be very careful that you're removing only the one object you wish to get rid of! I personally prefer the other methods for deleting stuck files which have been discussed here previously, but offer this up as yet another potential solution.]

