AJAX :: ComboBox Input / When leave The Combobox It Adds To The Combox List?

I'm try to add a combobox to my form which i have done and populate it with the infomation i need but i want to stop the user from being able to edit the first 5 character in the textbox part of the combo box( 1 is this possible 2 am i going about it in the right way).

I have 3 columns fro a db to enter ips ie 123.456.7.89 at the start if each number i want (SE1)(SE2)(SE3) which i can do at the momment . So I get in the combobox list 3 items with (SE1)123.456.7.89 OR just (SE1) depending of i a result is returned from the DB. However i don't want the user to be able to edit out the (SE?) part of the sting in the textbox. When editing i've tried used the text change event to try and capture the change and make sure the string.length > 5 , but the event does not fire also when i leave the combobox it it adds to the combox list. If I refersh it goes back to how it should be with only the 3 items.