Garageband additional software download status in Mac
GarageBand app does the download (spinning up a com.apple.MAContentInstallation helper process), so to stop it - save your work in any open app documents and quit the app.

You should have been notified of this like so:



The downloads go to your user library, but before trying to clean that out - what happens if you reopen the app and watch the dialogs and/or see the options in the GarageBand menu Sound Library sub menu?