As a longer-term solution, I would suggest adding cross-posting as an explicit reason when flagging and/or closing questions. This is already being used on Unix & Linux, listed under Off Topic.
This question has been posted on multiple sites. Cross-posting is strongly discouraged; see the help center and community FAQ for more information.
It seems to work well. Off-topic, cross-posted questions are generally closed soon after I flag them as such.
In the short-term, this suggestion doesn’t help those of us (such as the OP and myself) who have less than 3000 reputation and can only flag questions but I think it would be useful to add this feature. Note: It would also involve up-dating the What topics can I ask about here? help page.
Here’s a non site-specific version that might be useful for custom close reasons:
This question has been posted on multiple sites. Cross-posting is strongly discouraged; see the community FAQ for more information.