How to stop can't connect to iCloud message from popping up
Preferences > Internet Accounts: Remove your iCloud account.
That dialog is particularly annoying and persistent. Providing a valid password is one way of stopping the dialog from coming back, though I find that when this occurs it is best to do the following:
- Log out
- Log back in
- Enter a valid password the first time this dialog pops up, OR enter the valid password in the iCloud page of System Preferences.
If I enter the password on the second, third, forth, etc. request, It seem that not all the things that wanted it get access to it.
If the problem is that you don't know the password, and have no way of recovering it, you might try deactivating or deleting the iCloud account from the iCloud page in System Preferences, then restart or at least log out and back in. You can always add the account again later when you have the correct information.
A temporary workaround to this annoyance is to kill the iCloud helper daemon.
Launch Activity Monitor app (in Applications -> Utilities), search for iCloud, select iCloud Helper, and click the Stop button.
It'll stop the dialog from appearing until the next reboot. Helpful if you’re servicing someone else’s Mac and you don’t know the iCloud credentials, which was my case.