Can I stop an emoji reaction once it's started?

At this time, the only way to stop an in-progress emoji reaction is to close or refresh the App. You can do this by clicking the three dot button in the top right corner and selecting "Refresh App" or "Close current app."

Adjusting the "Display Time" slider bar all the way to the left can shorten how long the reaction appears, but the shortest available display time is 3 seconds, and it does not stop an in-progress reaction immediately.