The Sohma curse is broken when Kyo Sohma, the one cursed animal who isn't officially a member of the Zodiac Animal Calendar, finds true love. The curse is broken in the Volume 22, when Kyo admits he does love Tohru. He breaks off his bracelet that keeps him in human form, and remains a human.
There is also a theory that after Tohru calmed Akito. Akito just let go of the curse letting all members of the zodiac break free one by one. So as long as the god of the zodiac thought that the zodiac members would always stay by their side the curse is in effect. But if the god lets go of the members the curse begins to lift one by one due to the fact akito possess the spirit of the god.