Thumbs-down emoji should do it. Copy Dluvian's reaction to my reply :p
I do like Amethyst's freeform unicode reactions...
Discussion
they are not as flexible, you have to preconfigure them, but nostrudel also has unicode emojis... i see that there is gif and jpg/png ones too, not sure which clients are making those? primal?
Yea I loooove custom emoji reactions! I made my own pack even.