Maybe there is a more direct way to do it, but the scripts below will delete the objects that are created for database diagrams.
I delete then because when I have no diagrams in the db and when I run Scriptio, it includes them in the script. They are marked as system objects, so the "delete" option does not appear in the Sql 2005 Management UI.
Note that this does delete any diagrams you might have in your db.
DROP PROCEDURE dbo.sp_alterdiagram
DROP PROCEDURE dbo.sp_creatediagram
DROP PROCEDURE dbo.sp_dropdiagram
DROP PROCEDURE dbo.sp_helpdiagramdefinition
DROP PROCEDURE dbo.sp_renamediagram
DROP PROCEDURE dbo.sp_upgraddiagrams
DROP PROCEDURE dbo.sp_helpdiagrams
DROP FUNCTION dbo.fn_diagramobjects
DROP TABLE dbo.sysdiagrams