help with adding a 2nd zone in bsmt

I installed a 3/4 inch looped forced hot water zone in the bsmt.The weil mclain EG series 4 one pipe steam boiler was originally for the 1st and 2nd floors.I installed the optional tankless water heater coil for the closed loop.Ihave been using this system with the added bsmt zone now for 4years.This is my question: Is there any way to prevent the steam from traveling to the 1st and 2nd floors when the bsmt zone calls for heat?I have the zone wiring correct,but it seems that when the bsmt zone comes on ,there is no choice but to let the steam travel to the 2nd zone,even though heat was not called for at the 2nd zone.thankyou for any help