Regardless of what duty Janeway may owe to Seven's wishes, she's also responsible for the safety of her crew. Handing Seven back to the Borg - who, let's not forget, had just turned on and tried to assimilate Voyager, in violation of the alliance they had agreed - would place every other member of her crew in immediate danger.
The only other alternative would have been leaving Seven somewhere where the Borg could pick her up, and the Doctor might have had a few choice words to say about that, considering that it was strongly indicated that she would die if her implants weren't removed post-haste.
The only other alternative would have been leaving Seven somewhere where the Borg could pick her up, and the Doctor might have had a few choice words to say about that, considering that it was strongly indicated that she would die if her implants weren't removed post-haste.