Bounties are designed to be an irreversible journey down one way street. Once added, they're final - you should be extremely careful when selecting the bounty amount and reason. You should be equally sure when awarding the bounty.
Moderators do have the ability to remove and refund a bounty at their discretion. However, it's extremely unlikely that one would elect to do so in these cases - there's simply no guarantee that the user would place the bounty again if refunded. We avoid short circuiting the system, barring compelling or extraordinary circumstances.
There's really not much you can do if this happens. Feel free to flag, but realize that the chances of us taking action aren't good. A grace period could be an answer to this, but the idea hasn't received much attention.