JavaScript - eliminar option en ON CHANGE

<select class="form-control" id="7_piso_num2" value="" name="7_piso_num2[]" onChange="select2()">

<option value="0"></option>

<option value="1">1</option>

<option value="2">2</option>

<option value="3">3</option>

<option value="4">4</option>

..........

<select class="form-control" id="7_piso_num3" value="" name="7_piso_num3[]" onChange="select3">

<option value="0"></option>

<option value="1">1</option>

<option value="2">2</option>

<option value="3">3</option>

<option value="4">4</option>

..........

function select2(){

var dato2 = document.getElementById("7_piso_num2").value;

var dato3 = document.getElementById("7_piso_num3").value;

if ((dato2 ==dato3) &&(dato2!=0)){

 document.getElementById("7_piso_num2").value =0;

}

}

function select2(){

var dato2 = document.getElementById("7_piso_num2").value;

var dato3 = document.getElementById("7_piso_num3").value;

if ((dato2 ==dato3) &&(dato3!=0)){

 document.getElementById("7_piso_num3").value =0;

}

}

function select2(){

var dato2 = document.getElementById("7_piso_num").value;

document.getElementById("7_piso_num2").options[dato2].disabled = true;

}

function select(){

var dato2 = document.getElementById("7_piso_num");

var dato3 = document.getElementById("7_piso_num2");

var dato4 = document.getElementById("7_piso_num3");

for(i=1; i< dato2.length; i++){

	dato2.options[i].disabled = false;

}

for(i=1; i< dato3.length; i++){

	dato3.options[i].disabled = false;

}

for(i=1; i< dato4.length; i++){

	dato4.options[i].disabled = false;

}

dato2.options[dato3.value].disabled = true;

dato3.options[dato2.value].disabled = true;

dato4.options[dato2.value].disabled = true;

dato4.options[dato3.value].disabled = true;

dato2.options[dato4.value].disabled = true;

dato3.options[dato4.value].disabled = true;

dato3.options[dato3.value].disabled = true;

dato2.options[dato2.value].disabled = true;

dato4.options[dato4.value].disabled = true;

}