I dont know if you can do that.

What can do however is remove all pre marian reform units and change all the marian reform units so that they no longer the reforms. At least that would probably be your best option.
What he sayd .. just remove the pre-marian units from the export_descr_buildings.txt file and remove the "and marian_reforms" from the end of the units that has it...
Also from the descr_strat.txt you should change the marian_reforms_enabled line .. just put ; in front of it ...
IIRC this is what I've done