You could try Amyuni PDF Creator .Net for this task. You could use the method IacDocument.GetObjectsInRectangle to retrieve all the "graphic objects" of your interest:
IacDocument.GetObjectsInRectangle Method
The GetObjectsInRectangle method gets all the objects that are in the
specified rectangle.
Then you can iterate all the objects in the page and delete those that you are not interested in:
//open a pdf document
document.Open (testfile,"");
IacPage page1 = document.GetPage (1);
Amyuni.PDFCreator.IacAttribute attribute = page1.AttributeByName ("Objects");
// listobj is an array list of graphic objects
System.Collections.ArrayList listobj = (System.Collections.ArrayList) attribute.Value;
foreach ( object pdfObj in listobj )
{
// if pdfObj is not in the collection of interest
// then call pdfObj.Delete();
}
Update:
Amyuni PDF Creator .Net version 5.0 adds a new method IacDocument.Redact which might also be helpful in this kind of scenario:
The Redact method retrieves all the objects of the specified type in
the specified rectangle, deletes them, and draws a solid color
rectangle at their place.
Disclaimer: I work for Amyuni Technologies