Defining e-mail spam is relatively straightforward: unsolicited bulk email.
So it's not just about pills, or adult-related topics, but phishing, lottery and 419s are included in this. Huge volumes are sent via botnets to help avoid detection.
UCE (unsolicited commercial email) can be borderline spam if little care and attention is taken to target the audience, imho. Especially if they were from harvested e-mails. Many don't even ask for an opt-in or provide an unsubscribe option. Some of the largest companies in the world are guilty of inadequate opt-out systems. If that's the case, I simply blacklist all e-mail from their servers for a set time period until they either give up, or update their system. 180days is my standard setting.
I doubt we'll ever get rid of spam entirely as many of the amateurs will continue with their efforts.
It's phishing, imho, which is the greatest threat, especially as there are still millions of people gullible enough to think they've won the lottery they never entered, or those that fall for the banking hack, etc.