The following answer explains how to do this in VBA by setting the Locked property on the PowerPoint Shape. However, when trying to do this in C# as a VSTO Addon the Locked property is not available.
The ShapeRange class doesn't expose the Locked
property. If that works in VBA then you may try using the late-binding technology to call hidden or private members. Use the Type.InvokeMember method to make such calls in .net applications.