In the 'Data Sources' tab of the 'Project Details' window, click the 'Delete' button beside the Excel file you wish to remove from the project.

The 'Yes' button will be greyed out for 5 seconds, just to reduce the possibility of accidentally deleting a Data-Source link.

Once the countdown has run, you can click 'Yes' to delete the link to the Excel file.

NOTE:
Deleting the link to the Excel file will remove ALL links between the document and the Excel file. Relinking even the same Excel file will not be able to restore these links. If you wish to change the Excel file to which the document is linked, please select 'Relink' instead of 'Delete'.

Once you have deleted the link to the Excel file, click 'Save' or 'Apply' to write the changes to the project.

NOTE:
If you click 'Cancel' at this point, the linked Excel file will not be removed from the project, and all links will still be present in the document. In order to fully delete the links to the Excel file, you must click 'Save' (or 'Apply').

Once you delete the link to the Excel file, the values from the last update of the document from the Excel file will still be present. It is only the links themselves that are deleted, meaning that any changes to the Excel file in the future will not be pushed to the InDesign document.