Well there are two methods, The "cleaner" and pretty method is outlined in Kael's
GUIDE.
The faster "sloppy" way it to just give it an impossible requirement. IE add the following:
Code:
<GameData>
<Units>
<Update>
<Where Type="UNIT_MECH" />
<Set PrereqTech="TECH_FUTURE_TECH" />
</Update>
</Units>
<Unit_BuildingClassRequireds>
<Row>
<UnitType>UNIT_MECH</UnitType>
<BuildingClassType>BUILDING_STONEHENGE</BuildingClassType>
</Row>
<Row>
<UnitType>UNIT_MECH</UnitType>
<BuildingClassType>BUILDING_IRONWORKS</BuildingClassType>
</Row>
<Row>
<UnitType>UNIT_MECH</UnitType>
<BuildingClassType>BUILDING_NATIONAL_EPIC</BuildingClassType>
</Row>
<Row>
<UnitType>UNIT_MECH</UnitType>
<BuildingClassType>BUILDING_OXFORD_UNIVERSITY</BuildingClassType>
</Row>
<Row>
<UnitType>UNIT_MECH</UnitType>
<BuildingClassType>BUILDING_GREAT_LIGHTHOUSE</BuildingClassType>
</Row>
<Unit_BuildingClassRequireds>
</GameData>
Which means a city would have to have the ironworks, oxforduniversity, national epic, STONEHENGE, and The Great Lighthousein it and the Civ would have to have future tech researched befiore it could build a Robot.