i have a biter... help!!!!

well it looks like Amelia is turning into a biter . i caught her biting her one yr old cousins finger very hard the other day and he had done absolutly nothing to her. we were in the supermarket so i couldnt put her in time out but i made it very clear what she was doing was naughty and she now walks around the house saying "no biting!!" . i thought that was that until i went to preschool to drop her off this morning and her teacher said she had a complaint from one of the other childs parents because Amelia had bitten him more than once and left bruises on the poor boy . I cant describe how embarassed i was to hear this altho i do think preschool could of told me earlier as i had no idea . I must admit this is a wee boy that i often see hitting other kids and has hit Amelia on a number of occassions so it could be out of retaliation but thats still no excuse , i do not condone biting at all!!!! . i just dont know what to do . any ideas ? shes 20 months old and when i put her in time out in her bedroom for a minute she just tries to climb out her window or she just sits down and plays with her toys hapily so i dont really see it as a punishment . 

Here you go link. I hope it helps