I have a small blog page where registered users can login and comment. What I'm trying to find is a way to turn-off the form parameters for name, email, etc.

I only want the registered user to be able to add comment and NOT change any other information (name, email, etc) as part of the commenting form.

Hi. Name and email fields are required by K2. You cannot change this. You need to be able to write Joomla! plugins to alter this behaviour.

is there a way to disable the comment form user from changing the user name & email address?

What do you mean? You can make some hidden inputs in a template override. But then all your comments will appear coming from the same user. Another solution is to enable comments only for registered users. When a user is logged in he cannot modify those fields. Finally if K2 comments system doesn't fit your needs you can take a look at some other extensions providing comments for K2.

The second part is what I'm trying to accomplish. Only have registered users able to make comments, and not have user name/email modifiable. I thought I setup the module to only allow registered users

Is there somewhere else I have to set the comments from registered users only?

K2 comments options can be found under K2 Parameters.