You've not stated how you're sending your e-mail. (Outlook Express? Outlook? Thunderbird? Some form of webmail?).
I'll guess that you're using Outlook Express (although this will probably apply to Outlook as well).
The auto-complete function is only offering you a suggestion. If you continue typing the required address, the software will realise that you don't require the old one and let you enter the new one.
However, if you want to turn off the auto-complete feature, go to Tools > Options. Click the Send tab. Click to remove the tick from alongside 'Automatically complete e-mail address when composing'. Click 'Apply. Click 'OK'.
Chris