Interesting, I don't think any had thought of it, but I don't think there is much to be done in regards of it, neither do I think it is actually needed.
Perhaps placeing a couple of empty lines in the start of the post could minimize this.
But nevertheless - if you have put the spoiler text in black (applaud, many could learn from that

), and wrote spoiler in the thread title to mark it as a spoiler thread, then you have done pretty much everything you "should" to avoid people getting spoiled if they don't want to.
If people still can read some of the post by hovering over a spoiler post, then they wanted the spoiler anyways, so I think there is nothing to worry about in that regards.