I'm 99% certain units can walk through each other, there just has to be an available hex, and enough movement points, in order to do so. A wall of builders around your territory wouldn't do squat, unless that wall was at minimum 2 hexes deep, likely impossible due to costs. 6 builders around a city to prevent religious spread would be doable, but then the AI would just target another city, which imo, would just cause a lot of annoyance to try to bounce your builder wall from city to city, the grief to game the system wouldn't be worth the trouble. Of course, I'm sure plenty of people would be upset, though they would never do it themselves because of how annoying it would be for them, but would be upset anyway since it's possible to do.
Pretty sure you could this in CiV too, wall off a city with workers to blog missionary spam, but I've never tried it personally. But hey, there's a use for all those workers sleeping around my empire after all!