Simply put, you cannot stop all cheating, and what little can be reasonably done has been done. In addition, without evidence no one can be accused of cheating. Therefore there will be cheaters and their identies will likely remain unknown. In my experience with most games, however, the best players frequently can outplay the best cheaters, and even if the cheaters come out on top it simply serves to add to the fun and challenge of those not cheating. What greater satisfaction could you have in getting the top score knowing you had beaten all of the cheaters, with pure skill (and some good luck!)
I would even argue that the cheater is themself often robbed of the same feeling of accomplishment, particularly in a game in which cheating is so easy. Extensive anti-cheat mechanisms can actually encourage cheating in a way, and I have seen that in certain games as well. Ironic, but the cheaters sometimes feel that bypassing the attempts at catching them is a feat among itself. In Civ 4 this is clearly not the case and cheating would simply signify a lack of ability or skill, and an inability to admit to loss.
I would even argue that the cheater is themself often robbed of the same feeling of accomplishment, particularly in a game in which cheating is so easy. Extensive anti-cheat mechanisms can actually encourage cheating in a way, and I have seen that in certain games as well. Ironic, but the cheaters sometimes feel that bypassing the attempts at catching them is a feat among itself. In Civ 4 this is clearly not the case and cheating would simply signify a lack of ability or skill, and an inability to admit to loss.