Prohibit saving a post without providing a title

Perhaps these plugins will help you:

Mandatory fields

This plugin is responsible for making certain fields mandatory before publishing any post in wordpress.

Force Post Title

This is simple plugin which forces the users to write POST title in Add New Post page.

They both seem to prevent the posting only via JavaScript validation in the backend. If you want to restrict creating posts on the backend side you might want to look into these hooks:

• save_post
• wp_insert_post