Please try with the below code snippet.
public void Page_Load(object sender, EventArgs e)
{
GridFilterMenu menu = RadGrid1.FilterMenu;
int i = 0;
while (i < menu.Items.Count)
{
if (menu.Items[i].Text == "IsNull")
{
//Upadte Text
menu.Items[i].Text = "your_custom_string";
}
else if (menu.Items[i].Text == "IsEmpty")
{
//Rmeove menu item
menu.Items.RemoveAt(i);
}
i++;
}
}