laugh stock, laughing-stock, laughingstock, laughstock

Have seen all of them in formal writing.

But since "laughstock" is yet to appear in the almighty m-w, it is probably still somewhat on the avant-garde side. "Laugh stock" is certainly safe for now; in time, though, just as "laughing-stock" led us to laughingstock, it is only a matter of time when "laughstock" becomes more widely accepted and gets picked up by mainstream lexicographers.