I suggest expanding the list of sites in the Close Vote reason Belongs on another site on the main site. That will at least give the 'offenders' more information than Blatantly off-topic (which they would have to read the tour for to understand why - and we know how much people read).
It would still require discipline from close-voters to distinguish questions that would be off-topic 'over there' as well (a bad question is a bad question).