You could make them images.

Hi Katie

As Echo says converting to images will make it very difficult to modify
slides but AFAIK there's no simple way to lock individual "normal" slides. If
you just need to disuade people from modifying slides then this will make it
quite tricky but not totally foolproof.

Create a blank slide (layout blank) and zoom out to 10% view. Insert a
rectangle as large as possible and set it to "no line" and 100% transparent
fill. Make a few copies to increase the difficulty! Set the view back to
normal size. Now ctrl A to select them all and copy. Paste over any cpmpleted
slide that you do not want changing. The illusion is one of no shape being
selectable. If this is adequate you can easily automate the addition and
removal of the shapes with vba code to enable authorised edits.

Note that neither method will prevent slides being deleted.