- A new page specifically for submitting, up / down voting and reporting spam tags.
This would make most sense for you as this would be an incentive to open an account on blockchain.info. Up/Down-voting could be done by "donation" addresses which again would be good for you.
As long as you show the amount of donations as justification to rate something high, you should not be blamed for misrepresenting stuff and upon clear abuse you could close accounts or users and mark the tag as coming from a closed account. Showing tags from closed accounts last, only optionally or not at all would also be very clear to the user.
- Have an address that you can send a transaction to which contains a tag embedded within one of the scripts. This is abusing the blockchain a bit though.
As you said, this would be a very temporary thing and should be avoided for the sake of bitcoin.
- json file which you can upload to a webserver which will blockchain will index. Downside is you can't tag other people's addresses.
You would still need a way to get aware of these files. I see this approach as the most open one although people might be more careful with scam accusations due to the real world identities linked to their servers which of course is good for your liability.