Removing options from single accounts

Is there a way to remove certain options from accounts?

I have a test account, which users can use to see how the system works, but I don't really want them to be able to change the password or actually post comments/posts/pages. It is meant purely as a way for them to see the system in action before signing up for an account.

I suppose the simplest thing, would be to somehow buttons such as 'update profile' and 'post', though it would be good for them to have 'preview' button still working.

Is this possible?