Document maps only show in a PDF when you've set a document map label property on some report items. If you removed all of those it wouldn't generate.
Or you can (in HTML) add a flag to the URL which will stop it from showing but it's not clear if it works the same way for XLS: &rs:Command=Render&rc:DocMap=False
The general info I've said above is here: http://msdn.microsoft.com/en-us/library/ms156383.aspx