How to publish an irrevocable prediction or pledge?

* A predictor is identified by their e-mail address. It is assumed that a predictor has full and sole control of an e-mail address: only they can receive e-mails. (E-mail addresses can be also tied to identities on keybase.io, but that's beyond the scope of the question.)

* A predictor can submit a short prediction of a few words, about once a day. When making a prediction, (email, timestamp, text) is saved, and will be displayed in subsequent lookups. The timestamp is assigned by the server to the submit time.

* Predictions are public. There is a website where anybody can look up all predictions by e-mail address.

* The prediction database is append-only. It's not possible to edit or delete predictions after they were submitted. (It may be possible to comment on or to attach tags to existing predictions, but this is not a requirement.)

Is there an existing service or database (maybe a blockchain?) which could conveniently host predictions according to the requirements above?

One use case is following: I want to build a reputation of making many correct predictions and very few incorrect ones in a specific topic. Let's suppose that my e-mail address is well-known for people whose opinion I care about. I want to keep publishing predictions about once a week, and after a few years I want to make it easy for others to check what I predicted and when. It should be hard for me to cheat, e.g. by making both predictions (i.e. 1. Travel to Mars by 2025. 2. No travel to Mars by 2025.), and later deleting those which didn't come true. Thus Twitter is not a good solution, because I'm able to delete some of my tweets.