If you are using our email form processor (/store/emailformprocessor.aspx) as the action of your contact form. Add this field to your form. It will be hidden with javascript.
<input type="text" name="txtCheck" id="txtCheck" value="">
The script must be placed after the input field
<script type="text/javascript">document.getElementById("txtCheck").style.display = "none";</script>
The idea behind this protection method is that most spam/bots simply fill in any and all fields with random text and/or links. The email form processor expects this field to be empty. If it is not empty it was most likely filled in by a bot.