the registration page created by the security assist wizard will add the Dreamweaver check new User name behavior to the page to accomplish this.
There is not a Security Assist replacement behavior, but it is possible to accomplish using the a recordet and the Server Validation included in CSS Form builder.
1) Create a recordset that is set to filter the email column on the email form element.
2) Open the server behaviors panel and click the plus button, select WA Validation Toolkit -> Server Validations
3) Choose number validation
4) Enter "$totalRows_RecordsetName" (where RecordsetName is the name of the recordset created in step one), without the quotes, for the server variable, then click the plus button to add it.
5) In the number validation configuration screen, set the minimum to -1 and the maximum to 0.