thx for your help it was helpful, i find an other trick for a bigger area, when i was making a template for friendly server, i put a big object (pier concrete) i put in its init
this enableSimulation false;
this allowDamage false;
this setpos [(getpos this select 0), (getpos this select 1), -5];
this setVectorUp [0,0,1];
the object spawned under the terrain line and all the grass on that area totally removed.