To prevent LastPass from autofilling certain fields on specific sites or applications that you have control over, you will need to update your individual vault item as well as your LastPass browser extension preferences.

1. Edit your item(s) to add the 'data-lpignore'=true attribute within the specific form fields for which you do not want LastPass to autofill.
2. In your LastPass browser extension preferences (Advanced settings), enable the Respect AutoComplete=off setting, which will allow websites to disable the autofill feature.