Thanks to Rune FS's XML hack. (How to perform spell check and grammar check in Powerpoint using VBA?)

1. Save the pptx file as a PowerPoint XML Presentation.

2. Open the XML file with a text editor like 'notepad.'

3. replace every ' err="1"' with ''(remove any instance of ' err="1"')

4. Save the XML file.

5. Open the xml file in PowerPoint and save it as a pptx file.

6. Reopen the pptx file and all the red underlines are gone!