What is the religious ruling on a high school girl using a BlackBerry device, and would the guardian be sinful for preventing her from using it?

There is no harm in purchasing devices that may be used for good or for evil, and using them in a permissible way. If it is feared that the girl might use the device for something forbidden by Islamic law, and you are responsible for her, then it is obligatory to prevent her. For this, you will be rewarded, because she is under your responsibility. The Prophet, peace and blessings be upon him, said: "All of you are shepherds and all of you are responsible for your flock... And a man is a guardian over his family and is responsible for his flock."

It is obligatory to forbid evil by hand for those who have authority, while striving for gentleness, wisdom, and kind words.