// prevent drawing new lines
addMissionEventHandler["Draw3D",{
  {
    if (markerShape _x=="POLYLINE") then {
      deleteMarker _x;
    }
  } forEach allMapMarkers;
}];

// delete current ones
{
  if (markerShape _x=="POLYLINE") then {
    deleteMarker _x;
  }
} forEach allMapMarkers;