call UnitRemoveAbility(whichunit, 'aloc')
'aloc' is the raw data for locust, however if I remember correctly this doesn't actually change anything as the game engine treats units with locust differently than others
such that removing the ability later on with UnitRemoveAbility doesn't actually do anything to the unit (I may be wrong on that though)
Don't think it's actually possible to remove locust fully from a unit