You cannot erase pdf metadata with exiftool (it only appends your changes). To delete pdf metadata, you'll need gs.

Make a text file called 'pdfmark.txt'.

1[ /Title ()
2/Author ()
3/Subject ()
4/Creator ()
5/ModDate ()
6/Producer ()
7/Keywords ()
8/CreationDate ()
9/DOCINFO pdfmark

Then run:

1gs -o output.pdf -sDEVICE= …