If you have removed the association between a PPM project and a Microsoft Project file, resource mapping is retained (in case you want to re-associate the files).
You can manually remove the resource mappings from the unlinked file. After manually removing the resource mappings, if you re-link the file, automatic resource mapping is performed.
To remove the resource mappings from the unlinked file, do the following:
Open the project in Microsoft Project.
Locate the column that contains the PPM IDs used for mapping. The name of the column is set in the Microsoft Project Integration policy by the field PPM resource information requires one Microsoft resource custom field (by default, the column is named Number20).
Delete all the PPM IDs from this column.
Save the project.