I don't think you can... I don't see an "empty trash" button anywhere though. You can't permanent-delete a file just by removing it from a folder on your computer. You have to log in to the website, right-click it, select "delete", click the trash icon at the top of the screen, right-click the file again and select "permanent delete". That's not even close to what happened to OP.
The complaint is that Google Drive's UI is confusing, and can lead to unintentional data loss. Dropbox has a much better UI, and it's harder to lose data by accident.