1, Press ‘Command + R’ when power up.
2, Open Terminal and input command to disable protection.
csrutil disable
3, Login into OSX system, locate to /System/Applications, remove (rm -rf) applications do not use. ex, iBooks.app, Stocks.app
4, Re-mount root dict when can not remove or delete.
sudo mount -uw /
5, Restart and go into recovery mode (Command + R). enable protection.