Update:

this would be the javascript, to be executed before submitting the form:

	try{
		frm.anti_spam.value = "4599843126548";
	} catch(e){
	}
	document.formcontact.submit();

 this is the hidden field

<input type="hidden" name="anti_spam" value="1">

this would be in the handler:
 

anti_spam = Request.Form("anti_spam")
If anti_spam =  4599843126548" Then
    'go on with form processing
Else
    'spam attempt: stop processing
End If