Permanently remove icons from Dock

I have Maps, Photos and iBooks icons returning to my dock after a reboot even though I removed them several times already. Is there some way to remove them? They seem to be set as defaults, can I change these defaults somehow?

This answers it partially. When I encountered the same problem on El Capitan, these steps solved it.

1. Disable System Integrity Protection.
2. sudo /usr/libexec/PlistBuddy -c "delete:add-app" /System/Library/CoreServices/Dock.app/Contents/Resources/com.apple.dockfixup.plist sudo /usr/libexec/PlistBuddy -c "delete:add-doc" /System/Library/CoreServices/Dock.app/Contents/Resources/com.apple.dockfixup.plist
3. Re-enable System Integrity Protection.

I referred to another discussion for PlistBuddy