I bought some tiles somewhat close to one of the AI's cities. He called me up to complain about it and ask me to stop. I told him "my affairs are none of your business". But I had already purchased all the tiles near him so I didn't do it again. 50 turns later, he calls me up again to complain about me buying tiles near him (I didn't buy anymore) and ask me to stop. 50 turns after that, he pops up again. This was even a problem in Civ V for years so it is rather disappointing it didn't get fixed, although at least he isn't complaining that I broke my promise not to buy tiles this time.