I avoid building farms, health buildings, and anything else that increases population growth. I set my taxes so that I will get at least 0% growth rate. Then I start building units to get the city to the size I want.
Only the palace buildings tier are population dependant. Once you hit 24000, build the Imperial Palace and then exterminate the populace at your leasuire. The other final tier buildings do not look at the population, but rather wiether or not you have an IP. This way you can exterminate all you want and still build Siege Engineers and Hippodromes with 2000 population, provided you have the IP, of course.