Yes. But it's impossible to undo the things you've seen before. To cover up a spoiler, after you've clicked on it, you'll have to exit the channel and then return. However, the process of clicking on a spoiler after having exposed it won't be a success. Select a different channel or Server in an option on the left, and then go back to the same channel. When you enter the chatroom, you'll notice that the spoiler is hidden.