Yes, you can clear all data synced on Lightroom Mobile.
Open Lightroom CC Desktop version >> Go to Preferences >> Lightroom Mobile >> Delete all data.
Restart Lightroom CC & Mobile Version
Once done, Now you can only add those images which you want to sync.
Let us know if that helps.
Thanks! How do I remove the "Lightroom Photos" collection?
Delete a collection or collection set
When you delete a collection, no photos are removed from the catalog or deleted from disk.
In the Collections panel, do one of the following:
Select a collection or collection set and click the Minus icon (-).
Right-click (Windows) or Control-click (Mac OS) a collection or collection set and choose Delete.
um... I don't see a collection named "Lightroom Photos" in the Collections panel. What am I missing?
Look the collection folder under the collections panels and follow the rest.