For about a year now, we have a filter that warns and tags edits from "new" users that create a page in the main space. It has successfully identified page creations that should be removed, which only 2 exceptions (out of 2007 results total), one of which was a WMF user, and one edit that was moved to a sub-page in the creating user's userspace.
I'd like to propose to let this filter prohibit the creation of such pages, given its success in identifying pages that shouldn't be in the main space.
I'm not intimately familiar with AbuseFilter but I'm sure an exception can be made for WMF staff that haven't yet made 10 edits here so that they don't get caught in the crossfire.