Try this:
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="ddl1" runat="server" onChange="onChange();" AutoPostBack="False">
<asp:ListItem></asp:ListItem>
<asp:ListItem>Unpaid</asp:ListItem>
<asp:ListItem>Paid</asp:ListItem>
<asp:ListItem>Delivered</asp:ListItem>
</asp:DropDownList>
</div>
</form>
<script>
var ddl = document.getElementById("<%= ddl1.ClientID %>");
function onChange() {
var selectedIndex = ddl.selectedIndex;
var selectedOption = ddl.options[selectedIndex];
for (var i = 0; i < ddl.length; i++) {
if (ddl.options[i].value == "Unpaid" && selectedOption.value == "Paid") {
ddl.options[i].disabled = true;
break;
} else if (selectedOption.value == "Delivered") {
if (ddl.options[i].value == "Unpaid" || ddl.options[i].value == "Paid") {
ddl.options[i].disabled = true;
}
}
}
}
function disableOption(i) {
ddl.options[i].disabled = true;
}
</script>