Replace the select list:
<select name="UserState" id="UserState" onBlur="if (document.getElementById('stateServerValidation')) document.getElementById('stateServerValidation').style.display='none'" onChange="if (document.getElementById('stateServerValidation')) document.getElementById('stateServerValidation').style.display='none'" >
<option value="" selected="selected" <?php if (!(strcmp("", (ValidatedField("usersRegistrationfull","UserState"))))) {echo "selected=\"selected\"";} ?>>Choose a state</option>
[...etc...]
</select>
with a hidden form element of the same name, with the value set to "n/a":
<input type="hidden" name="UserState" value="n/a" />