Delete it from AD with ADSIEdit.msc
Run ADSI Edit & expand following items:
Right-click the Exchange 2003 server object which you want to delete, and then click Delete & click Yes in every adsiedit dialog box that prompts you to confirm the deletion.
Run ADSI Edit & expand following items:
Code:
Configuration Container -> CN=Services -> CN=Microsoft Exchange -> CN=<Your_Organization_Name> -> CN=Administrative Groups -> CN=<Your_Administrative_Group_Name> -> CN=Servers
Right-click the Exchange 2003 server object which you want to delete, and then click Delete & click Yes in every adsiedit dialog box that prompts you to confirm the deletion.