Had somebody report me for taking a health pack in ARAM

Depends really. If you're nearly full and your ally is low and you're intentionally taking away that resource for your team, then it's called "assisting the enemy team." Basically it's no different from ignoring your team and letting them die without you, or intentionally running into deaths to feed the enemy team. You're basically playing against your own team.

But denying a health pack is something you're unlikely ever going to get banned over. The new system has barely begun recognizing intentional feeders, it's unlikely it will ever be able to detect griefing. This is of course assuming you were griefing, in case you weren't you still have nothing to worry about.