Currently there is some debate about Lebensraum land tunneling. Personally, what I find most exploitative about it is the fact that when it's done during a peace treaty, there is pretty much no way to fight it. The enemy civ cannot even keep units near their cities due to a lack of open borders with you. If the two players were at war, this would not be the case. I believe this would keep the fun aspect of the policy while removing the most egregious exploit related to it, and would not be memory intensive at all, since all that needs to be checked is whether the citadel would steal an enemy civ's land and if so, whether there is a peace treaty.