Hi,
If you change user password, I think that he can reset it via 'Lost your password link' with WordPress help.
User should have at least 'Contributor' role to write post for your blog. You can change his role to the 'Subscriber' or your own self-made empty user role via user edit menu, and he will could not posting.
More general decision is available via 'authentication' filter you can add to your theme function.php file.
Check this post
http://www.johnkolbert.com/wordpress/how-to-add-your-own-authentication-criteria/
Example there checks the user name, but you can check if user has some role assigned and block him from login if such case has place.
Role checking for the user data is shown here
http://www.shinephp.com/how-to-block-wordpress-admin-menu-item/
Let me know, if you need help with a code for this case.
Finally, you can allow commenting to the registered and logged-in users only. Thus you can make a full block for posting and commenting for unwanted registered user as he could not login to your blog then.
Regards,
Vladimir.