Security

I have a scanned document which I need to redact (edit by blacking out). This is easily done by drawing a text box over the information in question and filling it with black.
 
My problem deals with preventing someone else (this document will be e-mailed) who is using Acrobat Professional from removing the text box.
 
It would appear that changing the security setting to disallow changes in the way to go but I cannot see where to change the security setting. That is where I need guidance.
 
Thanks.