There are a few ways you can do it that are more user-friendly than allowing the user to think they have requested a password reset. WordPress has a filter allow_password_reset see documentation
Simply add the following to your theme functions.php file
function wp_disable_password_reset() { return false; }
add_filter ( 'allow_password_reset', 'wp_disable_password_reset' );
If a user attempts to reset their password they will receive the error message
Password reset is not allowed for this user