I have seen a very active user who accumulated lots of reps. But he lost all of it and got suspended due to irregularities in voting. What are different irregularities in voting and on what basis does StackExchange suspends people ? I think it's good for the community to know the different malpractices that people adopt and which can come under the scanner.
In Short: Vote up or down on questions or answers where you see fit because of the content of the question or answer. Do not vote because of the author of a question or answer, do not create multiple accounts to vote on other's or your own answers and questions. Don't abuse the bounty system.
There are automated processes and moderators or stackexchange employees can take manual actions to look into or warn/punish/revert malpractice. There probably are posts on other meta's or help articles that go into more depth on this. I do not feel there's much benefit of writing up a full overview of what's tracked and possible. That may implicitly expose loopholes :-)
In regards to the user that got suspended recently: there has been communication back and forth with him/her. He/she will keep the reputation that was earned honestly.