Plagiarism is not the only rule of the forum. You need avoid cheating, spamming and ban evasion etc.
You're need to know the only one reason why user get permanent banned is due to plagiarism (most case) or spreading malicious link, if he never plagiarism other people post with his main account, his alt wouldn't get banned due to ban evasion. AFAIK spamming will only get punishment for temporary ban, although there's a permanent ban for spamming but it's rare to happen or maybe no one.
Plagiarism is not the only offence that gets one a permanent ban. You can say that it's the frequent one because it's the one newbies and mostly non English native people commit.
If spamming gets one a temp ban and the person continues to spam after serving the temp ban, it could lead to pam ban.
Cheating doesn't have any relation with ban punishment.
It depends on your definition of cheating. It is a rule than when you are banned you should rather appeal than appearing with another account. So, if you are banned and you create another account, that is
cheating.
If you copy another person's work and pass it to be yours in order to appear more intelligent, that is
cheating.
Hope you get.