I have updated the folder structure. Actually, I had put it in the wrong folder. Not it is corrected. Thanks for the feedback.
Best plugin for this would be Contact Form 7
You can disable User Registration from Settings->General->Anyone can register. Uncheck this to disable register. This will disable new registration but existing users can login and access their profiles.
Also I would suggest to use Captcha in registration form to prevent spam form submit, If you are not willing to disable registration.
