A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
This is possible to a limited extent with XML Hacking. Here's my article on the subject: XML Hacking: Locking Graphics. This article assumes you are locking graphics, which is a more fruitful pursuit than trying to lock placeholders. You should also read Introduction to XML Hacking and Editing XML in OS X as background.
The issue with locks on placeholders is that the lock is not inherited from the Slide Master to the Layout, or from the Layout to the final Slide. Locks on placeholders are only effective when you provide a final slide in the presentation and apply locking to the placeholders on that slide. When someone generates a fresh slide from layout, the placeholders will not have any locks on them.
My article uses the noSelect parameter. This page shows a list of other typical locks that can be applied: a:picLocks This list will vary somewhat depending on the actual placeholder type that a lock is being applied to.