Spam in comment section on Google Play - what is more powerfull vote down or flag as spam?
I would say that as a general principle, spam and posts with offensive abuse should always be flagged.

Flagging brings them to the attention of someone who may be able to delete the post and the account of the poster.  

Downvoting leaves the possibility that the posts remain visible.  It may take a while for a flag to be acted upon but - no flag - no action.