John is quite right, but you can add a "Deleted" field to your csv file, give it a value of 0 or 1, to "remove" the use from that course. The user and their data remains within the course but is hidden. It also means updating your database via a flat file, so use one and one only flat file for everything like this you do. Information for this is here. There have been occasions where this has been valuable for me when I can undelete a user simply by changing that value to a 0.
General help
Removing users from a group with CSV
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