Help in handling multiple job offers

Hi this is my first time switching jobs I have 4.4 years experience in finance investment banking. I got offers from three companies. I'm pretty much decided on which one to join. How to let the other down ? All of them happened simultaneously so I hadn't really told them I had any offers (I did say I'm in early stages of the process)

I heard from someone that once offer is generated in my name, and I reject it. I'm barred from that company for life. Like I'm still allowed to apply but they will not hire me. Is this true.

Also does anyone know if Northern Trust allows early release if buyout is in play. From my peers, I have heard that 99% it's only done due to health reason or emergency and no other case.