How to stop can't connect to iCloud message from popping up In this century I thought pop-up windows were dying out. But no, the following...

This Mac can't connect to iCould because a problem with "aa@bb.com"

...window keeps popping up. How can I stop it? Even though I've already pressed "Later", after 1 or 2 seconds it comes out again! I am struggling for minutes to do things, and it is really annoying. 

I'm running macOS 10.11.5.

Thanks!
Preferences > Internet Accounts: Remove your iCloud account.