Please moderate your style in future, otherwise your entries will be locked and deleted
Show us a screen shot of what you are talking about.
IIRC the only time a prompt comes up to Sign In for syncing is if you don't have a catalog and collection synced with LR Web, LR CC Mobile, and you are starting a New Catalog.
Is this what you are talking about? You are starting new catalogs all the time?
I am pretty sure it will do this every time you open Lightroom even if you are not starting a new catalog if you have never turned on Sync. Indeed pretty annoying if you don't want this. It will disappear as soon as you simply enable sync on one catalog. You never need to sync anything even if you do or even use that catalog, so that's the only real way to get rid of the dialog as far as I know. Even if you don't sync, it only comes up if you start Lightroom, which you rarely need to do - just keep it running.
For a long time I never synced any catalog to the mobile/web system and never got that prompt unless I started a new catalog.
Another option is in the identity plate area click the little triangle and Pause Syncing.
How did this answer my question?
I start a new catalogue for each new job. I DON'T want all my jobs in the same catalogue. I don't understand why I need to be forced into this SYNC/LOGIN every time. It is NOT a service I am interested in and ADOBE SHOULD have a way to de-activate this. It is EXTREMELY frustrating.
I will screen shot it when it happens again (which will be sooner than later sadly)
You don't have to sync anything. Just Sync up one catalog, you don't have to include any images, and the prompt will stop.
You can also create a Sync catalog that has no images in it and sync that and the prompts will stop.
Have you thought about using Folders for each client and or job and having only one catalog? You can name the folder with the clients name and for repeat client jobs create a subfolder with the date of the job and a brief description.
Just create a dummy catalog. Log on to CC in it and turn on sync on it and never open that catalog anymore.
Note that we are just fellow users, not Adobe employees. We have no influence over Lightroom development. If you think that there should be a switch to turn off CC syncing altogether, go to feedback.photoshop.com and suggest it. I am sure there is actually already a thread on this there that you can add your voice to. Contrary to this forum, Adobe engineers actually look there for suggestions and bug reports.
Why are you Signing out. There is no reason to sign out of LR.
Once you create a catalog to sync with you don't have to Sign Out to open some other catalog.
All you have to do is open that catalog by going to File > Open Catalog or File > Open Recent and select the catalog you want to open.
No need to Sign Out, Ever, so no need to sign back in.
What exact version of LR are you using? Go to Help then System Info and copy/paste the first 10 lines of text into a reply.