You can long tap on the username of the thread you want to hide. It was not the most intuitive thing for me but I found it accidentally and maybe I can save you from the searching!
I hope they add an option to swap the long and short press actions. I’ve found myself accidentally tapping and collapsing a ton of comments today.
Feel free to post a feature request: https://github.com/dessalines/jerboa/issues/new/choose