I'm on a tablet, same thing. To mark the box, touch the word "spoiler" once. You'll see the small square in the corner. If not, tap the word once again. Once should be enough, if it doen't work, try holding for a few seconds. If you manage to mark the section, the delete button can be found on every keyboard above the backspace. See picture if you don't know where I mean.
Otherwise, have you tried what Not an Ookla suggested? Reloading and then clearing the answerbox should work just fine except that you'll lose everything you already typed.