If you right-click on a shape or text box and select the Set as Default Shape, or Set as Default Text Box options, PowerPoint will record all of the properties of the object, including fonts, to a theme file attached to the current slide master.
You can have 1 default Text Box and 1 default Shape per master. For any new shapes or text boxes you create on slides with the same master, PowerPoint will create them using the properties from your original. So, if you later reuse some content in a new presentation then these default properties (including fonts) will make their way into the new deck. This is one of 3 ways unwanted fonts get into your PowerPoint files.
You can use Slidewise to replace the fonts, which will replace it in all existing objects and also overwrite the defaults, so that any new shapes you create will use your new font too.
However, if you prefer to edit them manually you will need to delete any existing text boxes or shapes that have the old properties (Slidewise can help you find them all), and then overwrite the defaults with new properties that you are happy with.
To overwrite the defaults you need to set up a new shape or text box with the settings you want and then set this as the default Shape and/or Text Box. To set the default shape style:
- Select a slide on an affected master
- Go to Insert > Shape > Rectangle and click on the slide to insert it
- Select the shape (the font should be the one you want to replace)
- Change the font to your new font
- Right-click the shape and select 'Set as Default Shape'
- Delete the Shape
The steps are the same for text boxes, except you insert a text box in step 2, and choose 'Set as Default Text Box' in step 5.