If you are using Civ3XEdit (which came with Play the World), go to the menu option Map, then un-tick (uncheck) the following three options: Polar Ice Caps, Allow X-Wrapping, Allow Y-Wrapping.
If you are using Civ3Edit (which came with Civ3) then it can't be done. The only way around it would be to make a band of mountains, with ocean either side, pole-to-pole on one side of the map- then a civ on the east side of the map would not be able to get to the west side over the boundry providing you don't allow cities to be built on montains that is.
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-Sir T
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