1) Create a recordset that is set to filter the username column on the username 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.