How can I remove them from there?
Don't do a purge all otherwise you will get rid of everything.
All you need to do is when the purge dialog opens up is to expand it to show all the blocks and individually select the blocks you want to purge out of the file.
I too want to customise a Projsymbolstyle.dwg by purging unwanted blocks. Many of the unwanted blocks in this dwg that I want to purge are unavailable for purging. How do I make all the blocks contained in the PSS dwg available for purging. Help anybody ! ?