After you remove them you may want to turn off syncing so that it doesn't happen again. In the top left corner near the name of your catalog, there is a disclosure triangle, turn that to open and turn off Sync with Lr Mobile". You can also disable Auto-sync by going to the Library module and under the Metadata menu and uncheck the Enable Auto-Sync, in the Develop module you can uncheck the Auto-Sync option in the Settings menu.
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