AutoCAD Civil 3D will hang on to some missing external references and their existence may not be apparent. Missing references include missing DWG file, images, ESRI Shapefiles, Data Links, and Databases. In most cases you can edit or disconnect missing reference files in the External References Manager, but not always. Today I want to pass on how to delete Data Links in AutoCAD Civil 3D.
Vault Won’t Release Files with Missing References
Data Management tools like Autodesk Vault bring missing data into focus. For instance, Vault audits the list of external references and gathers them together at check-in time. If Vault can’t locate all of the reference files, it will not permit a parent file to be released. If you have broken links, the next step is to go fix or delete them.
XREF Manager Can’t Delete Data Links
People use Data Links to populate drawing tables with Excel spreadsheet data. These will show up in the External References Manager. If the reference is missing, the yellow exclamation dialog will appear next to the reference. Unfortunately, you cannot delete the reference from the XREF Manager.

Delete Datalinks in AutoCAD Civil 3D using the Data Link Manager
Previously I wasn’t altogether sure how to get past this problem until I came across this discussion. The Data Link Manager organizes all the Data Links. The command for this is DATALINK. You can update or delete the link by selecting it from the list, and picking Delete from the right-click context menu.

The link will disappear from the XREF Manager after it is deleted in the Data Link Manager. More importantly, Vault will allow you to release the CAD file (provided all of the other requirements are met).