After some thinking, I guess you wanted to disable "Auto Complete" functionality in the browser. Right? The user can do this by visiting
Tools|Internet Options|Content|Auto Complete. ----I am talking about IE.

I am not sure how to do this by programming ( using JavaScript or VB Script). This is a browser configuration problem. The user has the privilege to choose to remember all the usernames and passwords.
If you "brutely" disable the "auto complete", you overdo. I guess it would be a better idea to using an "alert" ask the user to disable "auto complete".

Technically, it may be easier to use VB Script to disable the "auto complete". I don't think "unsigned Javascript" can do that. Again, I suggest you consider if it is necessary for you to disable the "autocomplete" functionality in your user's browser. If you do so, I think you have reached your hand too far. ;-):

Sorry for having misunderstood your earlier postings.