Here's an answer from the PowerPoint FAQ at
http://www.rdpslides.com/pptfaq/
Problem
You've assigned a template/design to a presentation and want to revert to the
original blank design.
Solution
Follow the steps here to create your own blank default presentation. You can
then assign this template to your presentation to bring it back to the original
default blank style.
http://www.rdpslides.com/pptfaq/FAQ00245.htm
Create a default "blank" presentation with your own defaults
Background
Every presentation has a template - you can't actually remove the template from
a presentation, you can only apply a different template.
PowerPoint's blank default template is built in, though. It's not supplied as
a file, so there's no way to apply it directly.
Instead, you have to create a blank template as described in the link above.
Then you have a template file you can apply to your presentations.
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