I'm new to Powerpoint 2007, and I'm working on a PPTX file that was handed to me with a number of established themes.

At the bottom right hand corner is a slide number which I cannot select and delete. Page Setup will only allow a number. I cannot set it to blank. There is an option for turning off the title page number, but that only applies to a footer added now. It won't change the existing page number. It seems as if the number is a part of the theme background.

I'm not sure if I can get ahold of whomever created the original document. Is there a way to eport the theme or edit the theme to get rid of it?

have you tried Insert > Header & Footer, and unchecked Page Number? That will probably do it.
If that doesn't work... to get to the underlying master, go to the View tab and click Slide Master.

Try this:
Right click on any slide, then pick 'format Background'. Pick 'Fill' on the left hand column and then tick "Hide Background Graphics"... worked for me.

Alternatively you will often find that a slide number is included in the Slide Master template which applies it to all slides in your presentation. To remove the slide number from the master simply go to the View tab and choose Slide Master.
From the slide master layouts listed identify the one which you are using for your slides, e.g. Title and Content and delete the text box on the slide which has <#>

Let me know if you require more assistance with this.