Not sure if the suggestion below produces the same result as WobertC has posted.
Backup first of course.
Go to C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\ and delete (or rename) the Sync.lrdata folder. Restart Lightroom and it should try to reconcile your local synced data and the cloud synced data.
I am afraid that today, there is no possibility. I hope Adobe gurus are working to improve the sync process : 1 ability to select what you want to sync ( date range for instance) 2 cleaning the issue of indefinite syncing as you mention