I would just continue to downvote and close. We don't want to discourage new users, so a nice comment to suggest improvements would be nice, but if it is a repeat offender I think a flag or vote to close is sufficient.
Most of the problem questions have been closed, and I think the problem will go away with time.
I will answer my own question and want to suggest the following. Post another answer if you disagree.
Leaving a comment - anyone with at least 50 reputation can do this. If you see posts which could be improved: Leave a comment. Don't flag it.
Downvoting - anyone with at least 125 reputation can do this. If you see posts of low quality: Vote down. Posts of ...
Just downvote them, and if the answer is also full of typos or intelligble, flag as Very Low Quality. When users get downvoted in a large part of their posts, they get locked from answering automatically.