You get an error message saying that an email or landing page is in use when you try to delete or even unapprove it.
If assets are being being used or referenced somewhere else in Marketo you cannot delete or unapprove them. Before you can, there cannot be anything listed under "Used By" section.
Navigate to the place where it is in use and remove it from there.
Once all asset dependencies have been removed you will be able to delete or unapprove the asset.