Protecting questions is a powerful tool given only to Supervisors. This feature comes in handy when a Q&A is being repeatedly vandalized or when spammers are attacking a specific page.
But be careful. Once protected, a question cannot be edited or improved. So if you choose to protect a Q&A, remember to unprotect it once things settle down so that legitimate contributors can add their posts.
The ‘Protect from Changes’ tool is located under Question Tools on the left side of the page.