The bad news is that you can’t disable it through an Acrobat Preference setting. You can decrease the frequency via Edit –> Preferences –> Reading Panel Screen Reader options: Only read the currently visible pages.
Another option is to disable the plug-in responsible for this.
Go to C:\Program Files\Adobe\Acrobat 8.0\Acrobat\plug_ins
and rename either accessibility.api or readoutloud.api to accessibility.old or readoutloud.old resp.