I have posted a question a while ago, but I got no answers so I decided to put a bounty on it.
My question is: How I do I prevent someone gaining the bounty point with a bad / stupid / invalid answer on my question?
If you don't manually award the bounty to someone, the awarded value will be automatically cut in half and will go to the highest-voted answer that was posted after the bounty started, given that it has a score of 2 or greater.
So, if you really, really want to avoid seeing your bounty given to an answer that is bad, your option is pretty much only to downvote the answer, leave a comment about why it is bad, and hope others downvote it or at least don't upvote it to 2. If somehow it gets to 2 anyway, you'll have to be content with the fact that they're only getting half the rep for it.