the ultimate question, well not really
is the movie called Hard Ball (as imdb states)?
or Hardball (as my dvd, other sites, if i recall my movie ticket, critic reviews, and anything else states)?
I'm 95% sure its one word.
Filmratings.com has it as "Hardball". The official site also looks like "Hardball." So it seems to safe to say that it is probably one word.
__May God bless.