I noticed that my comment form text areas are expandable, by using the mouse on the lowerright corner of the text area.
Can this be disabled?
Is this related to the mootools and uvumi files in the ATA options directory?

I believe that is a WordPress feature, you will see the same thing in the backend - go edit a post and you will see the same thing where you put the post text.

You are right. It apparently is a WordPress feature. It is not in the edit post text box, but you van find it in a lot of places in the WordPress backend.

By the way: you can see it in the lowerright hand of the "reply to thread" box on this forum as well!

I added:
textarea{resize:none;}
to the CSS Inserts which disables the feature.

If you want users to only change the texarea in a vertical direction you would add:
textarea{resize:vertical;}

Hope that helps some people that have the same issue.