Can I tell Photos I never want to use it with iCloud?
I'm going to take a step back and redefine the problem:


You have an iPhoto library with active shared albums.
You have a Photos library that you don't want to use iCloud with.


This is not difficult, but you need to work with the system, not go against the grain, which is what you're asking. Even if you found a command-line "solution", it would not be guaranteed to work in the future and would likely be undone with the next Photos update.

Here's what you do:

On the Mac that doesn't support Photos, you leave iPhoto. You leave your iPhoto Library intact and use as normal.

On all other Macs, use only Photos. Create a new Photos library (hold Option as you launch Photos and click "Create New") and designate that library to be the Photos System Library. This means that all Shared Albums, PhotoStream, etc. will be synced to this library, and this library only. See https://support.apple.com/kb/PH21363?locale=en_US for more information.

Other Photos libraries, which are not the Photos System Library, will show you the following when you look at Photos > Preferences > iCloud:



Now you can update your Shared Albums from any of your Macs, the one with only iPhoto or the one with Photos.

On the Mac with Photos, the only change is holding Option when you launch Photos to choose which library to use each session.

This will accomplish what you want without needing to 'hack' the system.

==== earlier answer mostly trying to understand the problem better ===

iPhoto and Photos are iCloud enabled, but only one library can be the primary iCloud-connected library. As iPhoto is deprecated, Apple is trying to nudge you along to only use Photos and get rid of iPhoto.

Photos is the replacement for iPhoto. They were never intended to both be used. In fact, in some cases, Apple would invalidate the iPhoto application to force users to only use Photos.

Since both iPhoto and Photos use things like PhotoStream and other services through iCloud, I imagine that the two programs are getting confused and they can't behave properly. Frankly, I'm surprised you're not experiencing sync'ing or other problems since you are using both.

The first time Photos appeared, it went through the process of migrating your old iPhoto library to a new Photos library, but it left the old one intact. This allows the user to double-check that the migration was successful before deleting the iPhoto library, to free up space if not for minimizing risk of confusion ("why are my photos appearing there and not here?")

So, if I may ask, are the photos in both your iPhoto Library and your Photos library more or less the same, or do you have two distinct libraries?

If the former, then I think you should move to Photos and ditch iPhoto completely, as it's no longer supported and will likely only cause further problems/confusion.

If the latter, then I suggest that you upgrade that new, distinct iPhoto Library to a Photos library for the same reason as above and use the Option-key modifier while starting Photos to choose which library you want to use each time you launch Photos. And ditch iPhoto.

The short version is that as iPhoto is deprecated, unsupported software, the longer you continue to use it, the more likely you will have problems.