How to remove an email from a thread?

I am relatively new to mail (migrated from entourage) and it's grouping totally unrelated emails because they have the same subject (hey or hi being very common subjects) and i would like to remove some of the emails from a thread. How can i do this?

Second - is there a way to set it up so it only threads them together if the sender AND subject are the same (ie. original title or re: original title).

Thanks!