The simplest way is to disable the technologies you don't want. In the Civ4TechInfos.xml file there is a tag
Change the value to 1 for every modern/future tech you don't want to see: you (or the AI) won't be able to research them anymore.
Ideally, you would do that by creating a very basic mod that you would put in the BtS/Mods folder. It should have the following structure: YourMod'sName/Assets/XML/Technologies. In it, copy the BtS Tech infos file and change as stated. Launch the mod.
Or just change the original file at your own risk.
Downside: when you have finished your reduced Tech Tree, research points have no value anymore as you don't even have a 'Future Techs' series.
Minor: the disabled techs will still appear in the Tech Tree but in red. They will still appear in the Civilopedia too.