I just came back to GalCiv 3 after years away, and there is nothing in-game to indicate that population is maxed out at planet class.
It would be really nice if I could delete/destroy/deconstruct/remove all of the useless cities I have built. It would have been even better if I didn't have to come here to the forums to find out about a fundamental fact of gameplay.
The Intrigue DLC got me back, and with another new DLC out in a few days I am sure a few more people are likely to give GalCiv3 another play through. Stardock really needs to, at the very least, include this information in the in-game tool tips and should really block the player from placing useless cities. Placed in the correct arrangement, two cities, a farm, and a hospital give a population of 33. Add a market or anything else that gives an adjacency bonus to population and you are well over any achievable planet class in the game (that I have seen).