You cannot keep individual objects from being deleted. But this technique
might help:
Add the logo to a blank slide. Then File | Save As and choose PNG. When
you're prompted, opt to save only the current slide.
Then Insert | Picture | From File and insert that PNG you just saved. Now
the logo is part of the background. You can put this image on the slide
master if you want, or you can just use it on an individual slide. Use
Format Background | Fill Effects | Picture, or just Insert | Picture. Your
choice.