you will need to make sure that all of the existing records have an email address.
you should also create a new column for Registerred, set the default value to 0, this will be used to make sure they dont come back and overwrite the registration with a new one.
next, create a recordset to filter the email address column on the email address form element, then set the recordset to advanced view, in the bottom section expand the tdatabase and the users table, select the registered column and click the where button to add it to the where clause, find it in the where clause;
and add " = 0":
ANDRegistered = 0
this will look up the email address and be used to make sure they have not filled in the registration already.
Now add server validation from Form Toolkit to the page:
Select number validation. For the server variable, enter:
where <RecordsetName> is the name of the recordset you created.
set the minimum number to 1 and the max number to 100
This will allow the form to submit only if the record set returns a value.
Next you will need to replace the Insert behavior on the registration page with an Data Assist Update behavior to update the registration record, make sure to update the Registered column to 1.