Negative. If there is no posted bounty on any forum BEFORE you are killed by someone claiming there's a bounty on you, they're breaking the rules for sure. I'd ask them to refer to the pre posted bounty. Other than that I don't believe you can hire in game for bounties unless you're present, and the other ships join in at the request of the opposing party. And you have to be aware of it (see the conversation leading to that arrangement), can't be in secret. You can hire BHG when you have posted a bounty, or freelancers, whoever you work with, but again, must be up already.
If they can't show you the bounty, I believe you can report.