How can I remove the template / revert to the original blank template?
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.
Create a default "blank" presentation with your own defaults
Problem
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.