How do I mitigate the risks of user-generated data when not requiring authentication?

I'm creating a web project where users can add markers on a map. I'm dealing with a demographic who are used to instant-gratification. I want to keep the threshold for engagement as low as possible so I'm considering making user registration entirely optional. There are a couple of pitfalls I foresee but since I know I'm a bit of a pre-optimizer (which is not always a good thing), I'd like to hear your opinions on the matter.

I think some of the most important risks are:

  • Spam-sensitive: since you are not required to identify yourself, it would be easier to get away with spamming the map with markers. I've seen it happen to a friend's project. He ended up with markers depicting genitals covering most of France.
  • Untrustworthy: because the source for the data can be anonymous, the data could be seen as unreliable.
  • Non-human interaction: one of the things having authentication helps with (usually) is barring machine data entry / spam bots. Leaving the app 'in the open' could attract bots.

Things I have considered to mitigate these problems:

  • Use only a CAPTCHA for anynomous data entry. I feel this is the bare minimum to do something about bots. Preferably use Google's one-click "I am not a robot" to keep the threshold as low as possible.
  • Offer some form of perk when using an account vs. anonymity. For example, earn reputation for your data entries based on votes from other users. However, I doubt most people will care enough.
  • When entering data anonymously, keep the data around for only a temporary period, for example 30 days. The data gets removed unless it is confirmed by a registered user.

The project will mostly be used by data consumers rather than data creators. This is the main reason to make data entry as user-friendly as possible. Because every consumer would automatically qualify as a creator, I just need to convince them it's worth their time. Therefore, I want to waste as little as possible on account registration.

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