S.G.
I am that crazy mom that will say to my child in front of everyone "I am sorry sweetie, it's unfortunate that thier parents have not taught them how to share and be a good friend. If he/she does not want to share, than you can choose not to play with them." I expecially enjoy this when the parent is sitting within listening distance. If the parent isn't there, then I would do what another person suggested and tell you children they don't have to share with the children who don't share.
I have also said allowed so that other parents can here (explaining to my daughter) "it is just nice if you leave things at home that you don't want to share with others, and some times people just aren't nice" Parents usually get off their but and do something then, because they don't want to look like thier not parenting.
Good Luck