Not sure if you're just being snarky, but I don't think the issue was that people didn't want others to claim credit on Em-8ER so much as those messages actually were spam. First definition of "spam" on Google:
This is likely going to be an unpopular opinion, but while I understand the need to get the word out for Em-8ER it's also a bit ridiculous to pretend that people aren't going to be annoyed when you send out actual spam. You guys have been doing this for over a year now and I'm not surprised that people are flagging advertisements for a crowdfunded game sent by spambots on an unmoderated dead forum.
Popular, unpopular no such thing here. As with many things it's simply matter of opinion and goes equally both ways. I fully understand your concern and opinion on the matter and don't feel like we need to convince each other to think otherwise. Everyone is free to voice their opinions and I consider healthy philosophical pluralism and ability to discuss it without offensive or demeaning overtone as beneficial to communication and relations.
Benefits are clear with thousands of new members and crowdfunding going over the roof, but this also has a logical side to it. If you subscribe to email notifications on discussion platforms, you do so with knowledge that you'll most likely get plenty of irrelevant and unsolicited emails. By marking emails from generic senders like FF forums as spam, you are saying "I subscribed to this platforms, but I don't want to receive communication from it and others shouldn't either", that's how Gmail spam filter works. It's flawed logic, because you might have subscribed to it to receive it, but can't, because others decided for you instead of simply unsubscribing themselves. That's why you can see hundreds of people around saying they are glad they looked into spam folders. They wanted that email to land in inbox, but google denied their wishes on opinions of others.
Taking down imgur images did nothing to lessen number of messages, it just replaced relevant images people clicked to see with broken images. This was nothing more than a spiteful action. Also saying that this was going on for over a year is a bit harsh. To my knowledge 2 PMs were sent, first PM was received by a very small fraction of users, second was more widespread and targeted specifically to offer reward to eligible accounts, not entirely irrelevant. You get 20 times more useless emails if you subscribe to popular thread, do you also mark those as spam if thread's responders aren't staying on topic? Or do you just unsubscribe from it?
Let me just say many responded to those PMs and are grateful they were made aware of Em-8ER, given free credit and can take part in this project from the very beginning and everything comes at a cost, question is whether it's worth that cost and effort. Everyone can judge that themselves. Personally I think it was worth it and this project benefited from it while hurting no one.