The obvious answer to your opening question is "no, attribution is not necessarily sufficient to avoid a suspension". Accounts can be suspended for plagiarism. There are really three better and more relevant questions to ask:

• What constitutes plagiarism (a more complete definition)?
• Should plagiarism be handled the same whether it is obvious/blatant (e.g., failure to attribute), or more of a "technical violation" (e.g., attributed but a non-obvious variant such as listed in the Wikipedia article)?
• Should first offenses (especially of a technical nature and apparently unintentional), receive the full hammer of justice or something less, like an educational warning?

What constitutes plagiarism here?

Plagiarism isn't a simple concept. A key part of the definition is wrongful appropriation and representation as one's own original work. So it might seem like attribution and making clear that it isn't your own original work should be adequate. The complication is that it doesn't apply just to the words, it extends to the thoughts and ideas.

In a school setting, students are supposed to learn concepts and how to think about something. So teachers are looking for the student to express their own thoughts; it is the only way to assess learning. The student can support that by quoting others, but the objective is to express their own thinking.

In that setting (and others that involve creation of original work), there is a large range of copying that may not be obvious to others as being problematic. The Wikipedia article includes a list of 10 student practices that educators found problematic in their setting. It includes things like:

Providing proper citations, but fails to change the structure and wording of the borrowed ideas enough.

Relying too heavily on other people's work. Fails to bring original thought into the text.

Taking passages from their own previous work without adding citations.

The list also includes matters of attribution accuracy, such as:

Melding together cited and uncited sections of the piece.

Citing some, but not all passages that should be cited.

Learning how to write properly, and cite properly, is important in a school setting. It's also important in one's employment if that employment involves creating original work. The Stack Exchange strives for the highest level of quality and is very cognizant of fair use and plagiarism, we don't want the site to get a bad reputation or have to deal with outside authors who claim we lifted their work.

Handling blatant vs. "technical" violations

That said, we are not an academic institution. In fact, there is virtually no "original work" posted here (and it probably wouldn't be considered a good answer if it was because it would be hard to validate). Nobody gets upset if an answer repeats, without citation, material from the author's own previous posts (even though that could be considered plagiarism).

We can argue as to whether some of the practices educators find problematic are even all that relevant here. My own feeling is that we should separate, and handle differently, blatant and obvious plagiarism from practices that are more in the nature of technical or non-obvious violations, especially for a first offense.

Practices like "melding together cited and uncited sections of the piece" should be handled more like English mistakes. The same would apply to a large collection of mixed quotes, where there is an attempt at attribution, but not every quote is properly attributed. If it's discovered, we can educate the poster and get it cleaned up. If the poster has made an honest attempt at attribution, they should not get suspended for plagiarism.

Matters like quoting too large a portion of a work are a gray area. How much is too much? Where does fair use end? These are, to some extent, legal questions and matters of opinion. If we provide clear guidelines to posters, we have something that can be known by posters and enforced uniformly. Without posted guidelines, enforcement will appear arbitrary and capricious.

Suggestions

• Create guidance in the help section that clearly and in detail spells out expected practices.
• Most people know that posting someone else's work as your own without attribution is plagiarism. Perhaps there should be some discretion for handling a new poster (warning and education for the first offense if it appears to be simple lack of awareness), but this kind of blatant plagiarism is what most users would expect to be associated with a suspension.
• If the poster makes an honest attempt at attribution and otherwise avoiding blatant plagiarism, but still manages to violate a more technical rule, this should not result in suspension, at least for a first offense. The focus should be on educating the poster and fixing the post.