Id say no as "Fire for Effect" is a special order that the command squad issues in the command phase to any unit with the Artillery Strike ability and does not contradict or replace the Radioman ability. You would also have to use the Radioman abiltity to issue the order to an observer squad outside of command radius.
PG 88 "Once per Command phase, the platoon’s Command Section may issue a “Fire for Effect!” Order to one unit with the Artillery Strike ability (which may include itself). The unit receives a Reaction Marker. Then roll a single Combat Die, and on a hit the ordered unit may resolve a 220mm Long Tom attack against up to two target units. Each target unit must be within 6” of each other. The attack may fire indirectly, even though the unit did not take a Sustained Attack action."
In order to stop the Long toms you would either need to wipe out the command squad or kill all the Radiomen in the force (within command radius).