This is really a browser feature that you don't see in some of the older browsers. I know that for Chrome you can do the following:

In your CSS use this (for textareas):

textarea {
resize: none;
}

I can't guarantee that this will work for other browsers. It's not a setting in Concrete5, nor is it something Concrete5 can manage. You'll need to do a little research on Google, but here's some links I found:

http://www.electrictoolbox.com/disable-textarea-resizing-safari-chr...

http://brett.batie.com/website_development/no-resize-textarea-in-ch...