Solution
In Quark Publishing Platform 12.x (September 2016 Release):
1. Open 'xmleditor-config.xml' file located under C:\Program Files\Quark\Quark Publishing Platform\Server\webapps\workspace\editor\config folder in notepad++. (Please ensure to take back-up of this file before making any changes)
2. Go to the toolbar section and remove/comment-out the required button from the item list. For example, if you want to remove 'Undo' button, delete the undo item or comment it:
3. Save the file and open a Smart Document and verify the button no longer appear in the Editor toolbar.
In Previous versions:
1. Open 'editor-config.js' file located under C:\Program Files\Quark\Quark Publishing Platform\Server \webapps\workspace\editor\js\thirdparty\ckeditor-4.5.3 folder in notepad++. (Please ensure to take back-up of this file before making any changes)
2. Look for 'config.toolbar_XmlEditor' and remove the required button from the list. For example, if you want to remove Spell Checker, then delete 'Scayt'.
3. Save the file and open a Smart Document and verify button no longer appear in the toolbar.