In Firefox, you can use the up and down arrow keys to scroll through the autofill options and press the delete key to delete them. To delete all of them tap down twice then hold delete.

In Chrome, you can go to your history (ctrl + h), click "clear browsing data", switch to the Advanced tab, set time range to "all time", select "autofill form data", then press clear data. This will affect other sites as well. Nevermind that doesn't work. I'm not a Chrome user. I don't know how to do it in Chrome.