You are unable to archive or delete an asset in Marketo. When you try to do so, you get an error: "The following assets are in use or used by other assets."
To archive or delete the asset, you will need to determine where it is being used. Click the Used By tab for the asset you are trying to delete and see what other assets reference it in some way. For instance, aSmart Campaign may be using it in the Smart List or the Flow Steps, or a report might have it in the Smart List.
Once you find where your asset is being used, you can go and remove references to it from the other assets. This should allow you to delete or archive it.