How to stop syncing between accounts on iMac?

What, exactly, are you trying to accomplish?

Do you want to disable ALL sync'ing via iCloud in the second Mac user account or ONLY disable password sync'ing?

In the second Mac user account you "could" simply disable all iCloud sync items by going to System Settings > Apple Account > iCloud > See All and turn off all sync items; or sign out of iCloud entirely in System Settings > Apple Account > Sign Out. This will not affect your iCloud settings in any other user accounts on the Mac. Each Mac user account has its own settings.

margalina wrote:

If I sign the second local account out of iCloud, and create a new Apple account for that user, won’t that user then have lost everything in his account?

That depends on whether you consider that account his...

There is only one account. They are going to lose everything that was being synced to iCloud.

Separating the data from a "shared" account is messy. That's one of the reasons apple explicitly says that you should not share accounts.

Here's a good guide to get you started: https://discussions.apple.com/docs/DOC-250008248

You sign the second local account out of iCloud, then open a browser and go to https://appleid.apple.com and create a new account for that user.

Each person needs their own Apple account. they should never be shared.

if you have two local accounts on your Mac and you use both of them, and don't want the data to cross contaminate, then create a second Apple account for the second local account to use.

And what, exactly, do you think is syncing between them? Unless you're using the same Apple account for both of the local user accounts, nothing should be syncing between them.

margalina wrote:

I am using the same Apple account for both of the local user accounts

Well there's your problem.

Don't do that.

, and it is syncing my passwords. If I add a password in one local account, it also gets added to the other local account.

That is EXACTLY what is supposed to happen when you use multiple devices with the same Apple account. Or in this case, multiple local accounts on one device with the same Apple account.

The account that this user's data has been going to is your Apple Account. Having them create their own account, they would then save any new data to that new account.

If you are specifically talking about data that they have been saving when logged in as this "sub account," that data will still be on the Mac's internal drive as before. Them creating a new Apple Account to be used with their "sub account" shouldn't affect that data.

First, define "sub account".

Are you referring to two different user accounts on the Mac? Two different Apple accounts?

What exactly is being "synced"?

This is too vague to address without additional information.