This is it. Most people here were banned from subreddit; that's not really that severe compare to being banned site-wide. Anyway if someone keep reposting to the banned subreddit (even by using different accounts but same device), they'll get banned site-wide. And once they get banned by reddit, it's really difficult to sign up with new accounts. Even if they manage to circumvent system, they'll be a good chance that reddit will discover through one way or the other in the future (through device or IP, and they'll be re-banned. You seem to partially confirm this.

Once I got someone banned (not something I am proud of) because he misused the reddit suicidal report to report me. The funny thing was he actually wanted to report someone else but he mistakenly selected me as the reported party because I was replying to his comment under a different subreddit of academic/learning nature (r/whatstheword). I know it was him who reported me as suidical because my innocent comment was replied by him using swear words and not long after that I receive messages from reddit about where to get help if I felt suicidal. Anyway, once I reported him, it was quick. Around the same day or next, his username was deleted along with all his comments. I couldn't search his username any longer. He's vanished from reddit altogether.