If you ensure that data is only entered via a form, then you could do it in the Before Update event of the form and you can use a DCount to check to see if the value exists and if so, cancel the update and message the user.
If DCount("*", "TableOrQueryNameHere", "[FieldNameHere] = " & Me.YourControlNameHere) > 0 Then
or if the field is text:
If DCount("*", "TableOrQueryNameHere", "[FieldNameHere] = " & Chr(34) & Me.YourControlNameHere & Chr(34)) > 0 Then