You don't want strangers to be able to contact your kids. So advise your teens to tweak their privacy settings to restrict who can see info about them online, especially on social networking sites like Facebook. Also, work to limit how searchable their personal information is, because the less potential cyberbullies are able to access and exploit it, the better.
You can also block or sever the lines of communication between an existing cyberbully and your kids by having them take steps like changing their e-mail addresses, phone numbers and other forms of online presence. Remind them to never share their passwords or stay logged in to sites when they're away from their computers.