Unit disabled with scripts ?

Is it possible to disable the recruitment of a given unit with scripts ?

I'm considering having a early and late Milites unit for AoVaF, one with a round shield and the other with a kite shield and a segmented helmet. I already know it's possible to make some units available later in game, but what about disabling an unit ?

you can let a script create a building that allows the new unit to be trained, and disables the previous unit (using "not" in the "requires" line).