For form autofill data, you can goto the URL opera://settings/clearBrowserData, switch to "advanced", check just "autofill", set the time range to all time and delete the data.
You can also goto the URL opera://settings/autofill and click on "addresses and more", which will take you to the URL opera://settings/addresses where you can disable "Save and fill addresses" if you want. Disabling it might clear existing things too.
If all else fails, you can goto the URL opera://about, take note of the "profile" path, close Opera and delete the "Web data" file in the profile folder.
If you just want to delete a single entry for a form, when the entry shows up, you can hit the down arrow to highlight that entry and hit shift + del.