A kind of call centre job you might find yourself doing one day. Essentially, it's any job where you sit and take complaints, but don't have any authority to fix the problem or help the customer, so all you can do is sit there and sponge up all the hate and anger they're directing at the company or product. Hence, Hate Sponge.
Anna: "So I heard you're working at SOME CORPORATION now"
John: "Yeah, but I'm just a hate sponge. It sucks."