How do I configure the form validation to only accept letters for firstname and lastname?

I'm working in Dreamweaver CS6 and have a question in regards to the form validation. I already have a hidden input field in place and with the coding in my formmail script it has stopped spam over the years. At the moment however, I'm getting spam where the firstname and lastname fields present with a mixture of numbers and letters.  This spam seems to be only targeting my "Required" fields. Is there a way to specify that the name fields type:text be only letters a to z?

The form validation at the moment is:

<script type="text/javascript">

<!--

function MM_validateForm() { //v4.0

  if (document.getElementById){

    var i,p,q,nm,test,num,min,max,errors='',args=MM_validateForm.arguments;

    for (i=0; i<(args.length-2); i+=3) { test=args[i+2]; val=document.getElementById(args);

      if (val) { nm=val.name; if ((val=val.value)!="") {

        if (test.indexOf('isEmail')!=-1) { p=val.indexOf('@');

          if (p<1 || p==(val.length-1)) errors+='- '+nm+' must contain an e-mail address.\n';

        } else if (test!='R') { num = parseFloat(val);

          if (isNaN(val)) errors+='- '+nm+' must contain a number.\n';

          if (test.indexOf('inRange') != -1) { p=test.indexOf(':');

            min=test.substring(8,p); max=test.substring(p+1);

            if (num<min || max<num) errors+='- '+nm+' must contain a number between '+min+' and '+max+'.\n';

      } } } else if (test.charAt(0) == 'R') errors += '- '+nm+' is required.\n'; }

    } if (errors) alert('The following error(s) occurred:\n'+errors);

    document.MM_returnValue = (errors == '');

} }

//-->

If anyone can help it's greatly appreciated.