During review I come across “question” posts that don't actually contain a question. In some cases the question is implied in a statement. In either case it would be better to have an explicit question.
Mandate at least one question mark per question post (outside of quotation and pre-formatted listing environments).
Notify users of this requirement when they fail to comply with a short explanation, possibly with a link to “How do I ask a good question?”.
If (1.) is too strong, as a variant we could remove the mandate but still do (2.) and ask question authors to confirm that they don't want to use a single question mark.
This is no sure-fire way to reject non-question question posts but it serves to direct the attention of question authors as to the lack of an explicit request and guide them to use Ask Ubuntu as a Q&A site as intended.
I'm not trying to enforce “proper punctuation”; that undertaking would be entirely in vain. Instead I would like use the absence of even a single question mark as an indicator that the author didn't ask an explicit question because they are unaware of or misunderstood the Q&A model of AU/SE.