Is VisitorID the unique ID for the table? If so, it should be set to auto increment anyway.
However, if Visitor ID is another field in the table then the best way to do this is call a recordset on "Post" of the form against the values in the form, check to see if the recordset is populated. If it is then present an error and if it isnt then use Data Assist to insert the record.
I might be wrong but I dont think Data Assist will do all this for you - you need to do a bit of manual coding to acheive the result.