If you take a look at Journal Setup Step 4.1, you'll see the options for controlling how users register on your site. You can limit registration to the reader-level only, and further register readers as authors and/or reviewers at your discretion; I would suggest putting some sort of notice to that effect on your main page (which you can do from Journal Setup Step 5).
As a side note, OJS 2.2 includes CAPTCHA support for user registration and comments.
As per question 2, that is more or less correct. You can view the following thread for more information on deleting users: viewtopic.php?p=5109#5109
Further discussion on merging instead of deleting users can be found here: viewtopic.php?f=8&t=1970