I've done this and the OPs suggestion, and the incorrect autocomplete persists. In my case, I went to a random person's blog that starts with the same two letters as a website I use frequently.

I've given that random person's blog thousands of hits over the last year...

If it is in your top sites, you need to add that widget to the home page to get access to the menu to dismiss the result. (See my reply to the grandparent post for details.)

I don't know of an easier way to do this.