I think that you need to disconnect the client PC's (uninstall is probably easiest). Then delete the users from the Synology (after backing up any data if you want to keep it though the data is more likely to be up to date on the clients than the server?).
Then go and reset the cloud storage settings on the Synology.
I think that should work. Not actually had to do this so proceed with caution. Not much can go wrong if you have copies of the data somewhere.
Don't think you will need to worry about the logs as you've deleted the users and the user data.
Reread the Synology documentation to make sure you are following the recommended practices. Then start with new users then install the clients. Obviously, you will want to install the clients one at a time and start with a single shared folder before proceeding.
Also note that if you turn on full indexing, it can take DAYS at very high CPU levels. Take it slow and steady.