Hi Waghmare,

Refer below code.

HTML

<div align="center">
    <asp:ListBox ID="lstFruits" runat="server" SelectionMode="Multiple">
        <asp:ListItem Text="Mango" Value="1" />
        <asp:ListItem Text="Apple" Value="2" />
        <asp:ListItem Text="Banana" Value="3" />
        <asp:ListItem Text="Guava" Value="4" />
        <asp:ListItem Text="Orange" Value="5" />
    </asp:ListBox>
    <asp:Button Text="Disable Select All" runat="server" OnClick="OnDisable" />
</div>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link rel="stylesheet" type="text/css" href="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css" />
<script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
<link href="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/css/bootstrap-multiselect.css" rel="stylesheet" type="text/css" />
<script src="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/js/bootstrap-multiselect.js" type="text/javascript"></script>
<script type="text/javascript">
    $(function () {
        $('[id*=lstFruits]').multiselect({
            includeSelectAllOption: true
        });
    });
    function DisableSelectAll() {
        $('input[value="multiselect-all"]').attr("disabled", true);
    }
</script>

Code

C#

protected void OnDisable(object sender, EventArgs e)
{
    int selectedCount = 0;
    foreach (ListItem item in lstFruits.Items)
    {
        item.Selected = true;
        item.Attributes.Add("disabled", "disabled");
        selectedCount++;
    }
    if (lstFruits.Items.Count == selectedCount)
    {
        string script = "window.onload = function() { DisableSelectAll(); };";
        ClientScript.RegisterStartupScript(this.GetType(), "DisableSelectAll", script, true);
    }
}

VB.Net

Protected Sub OnDisable(ByVal sender As Object, ByVal e As EventArgs)
    Dim selectedCount As Integer = 0
    For Each item As ListItem In lstFruits.Items
        item.Selected = True
        item.Attributes.Add("disabled", "disabled")
        selectedCount += 1
    Next
    If lstFruits.Items.Count = selectedCount Then
        Dim script As String = "window.onload = function() { DisableSelectAll(); };"
        ClientScript.RegisterStartupScript(Me.GetType(), "DisableSelectAll", script, True)
    End If
End Sub

Screenshot