Downvote

[Kit]

to click (= choose) a symbol to show that you dislike or disagree with something you have read or seen on the internet:

[Julian]

a symbol that shows someone dislikes or disagrees with something they have read or seen on the internet:

[Couch Potatoe]

(in an online context) register disapproval of or disagreement with (a post or poster) by means of a particular icon.