Well there's only two ways to cut down on stealing technology, and both are conditional.
If this is for a multiplayer scenario you can simply edit the game.txt menu and remove the "Steal technology" line from the appropriate menu text - blank the line, don't delete it. This will stop the option appearing to humans but won't hinder the AI.
Number 2: there is a flag in the city information section of the save game which records if a technology has already been stolen. Diplomats won't be able to steal technology from that city, but spies will. And the flag is reset when the city changes ownership. Not perfect but it will slow it down. You can use the Civcity editor program to do this quite simply, unlike poor Harlan all those years ago doing this for his WW2 Europe scenario.
If you worried about them stealing special technologies and screwing up vital civ-specific units you may not need such a drastic option. We can give you tips on what to do.
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Ideological conviction quickly degenerates into insistence based on ignorance.
