There is somebody who created a UserScript for that purpose.
As for having that feature directly implemented in Stack Exchange, it will not probably be implemented, for the simple fact Stack Exchange focuses on questions and answers, not users. If you notice, it is not even possible to flag a user: To do that, you need to flag a post written by that user, and explain with a custom reason why you are flagging. It is not possible to make friend another user, in the same way it is not possible to follow a la Twitter another user.
In the specific, ignoring the posts written by a user will not probably implemented because that is contrary to the idea of community moderation. If everybody would ignore the same user, nobody would notice what that user does and flag any problematic posts written from that user. Even in the case just few users would ignore the posts written by a specific user, the chances of noticing bad posts from that user would be reduced.
As note about community moderation, notice that flagging posts in a small site like Drupal Answers is disrupting the idea of community moderation, since you are asking to the moderators (the recipients of the flags) to do something that would have immediate effect: If a moderator votes to close a question, that is immediately closed; if a post is voted to be deleted from a moderator, that is immediately deleted (without three or more users votes to close it).
Flags that don't use a custom reason are visible also to 10K users, but in the case of Drupal Answers, that would mean 3 users. If you look at when those users access the site, you will notice that in most of the cases, the flags are seen just from the moderators.
If the reason to flag is one of the following, then flagging for moderation attention is correct, since only moderators can take action on these cases.
- The OP posted as answer what was supposed to be an edit for his question
- A user posted as answer what should have been a comment (e.g. "I have the same problem. Did you find a solution?") or a different question (e.g. "I have a similar problem, but in my case [explanation of the differences]. How can I fix this?")
- A spammer posted spam
- A user, angry because his question has been closed, changed the question into an offensive post toward the community
Flagging a question to get it closed is a borderline case, as Drupal Answers have 14 users who can vote to close the question. I would rather flag a question when it is too old to be noticed from those who can vote to close, or when it is a very bad question, blatantly off-topic, constructive, or not a real question.
Maybe, in most of the cases, it is better to first leave a comment, if the post has not been written from a user who starts to write spam from the very first post.
Notice also that moderators don't close a question simply for the fact a user flagged the question to close it; they close a question if they think it should be closed. In fact, the question being voted to be closed from other user is never a justification for a moderator to close it.