Suppose you are a participant in an online forum where people review online games, and one participant recently posted an angry rant in all caps, which elicited a harsh rebuke from several other users. such a reprimand is a kind of?

Informal Sanction.

Explanation:

This kind of sanction doesn't imply a law itself, but rather express an individual dissatisfaction towards someone or something, the society sanctions this behavior in an attempt for the other to change it.

Some modern movies are boycotted by angry fans that are not satisfied with the product, and end up damaging the overall image of the movie, and the film company. This is a perfect example of social informal sanction.

because the early people found out about it.