Perhaps I've found the key to postponing colonization of Siberia. I've made a experiment and seen that AI sends a warrior asap to the nearest barbarian camp, and if there isn't a barbarian camp, to the nearest goody hut. Now, removing the barb units doesn't change this priority. Removing the camp and leaving the units instead changes this: the warrior heads to the goody hut, and ignores the barb. unit, which instead of being attacked, attacks the warrior when it comes near. For this reasons, limiting the number of the camps and raising the number of the units would mean that barbarians will last much longer.