I've created a no spam workflow and figured I'd just post it here to help others with a similar issue.

Basically, all this workflow does is check to see if a hidden field is modified from its default value and either deletes the record, does nothing with it, or approves it.

Set your form to manual approval for this to work, and move your workflow steps around accordingly.  This workflowtype should go inside the on submit event.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Umbraco.Forms.Core;
using Umbraco.Forms;
using System.Web.UI.WebControls;
using Umbraco.Forms.Core.Enums;

namespace Umbraco.Forms.Core
{
    public class NoSpam : Umbraco.Forms.Core.WorkflowType
    {
        [Umbraco.Forms.Core.Attributes.Setting("Spam Field Label Name",
            description = "The label used for no spam hidden control",
            control = "Umbraco.Forms.Core.FieldSetting.TextField")]
        public string NoSpamLabel { get; set; }

        [Umbraco.Forms.Core.Attributes.Setting("Spam Field Default Value",
            description = "The set value for the no spam hidden control",
            control = "Umbraco.Forms.Core.FieldSetting.TextField")]
        public string NoSpamValue { get; set; }

        [Umbraco.Forms.Core.Attributes.Setting("Action", prevalues = "Delete Record, Do Nothing",
            description = "What to do if it matches",
            control = "Umbraco.Forms.Core.FieldSetting.Dropdownlist")]
        public string Action { get; set; }

        public NoSpam()
        {
            this.Id = new Guid("31804252-AD24-4755-B040-2DE0282D893C");
            this.Name = "Perform filtering if the no spam field is modified/removed";
            this.Description = "Changes the state of the record being processed";
        }

        public override List<Exception> ValidateSettings()
        {
            return new List<Exception>();
        }

        public override WorkflowExecutionStatus Execute(Record record, RecordEventArgs e)
        {
            bool spam = false;

            RecordField nospamField = record.RecordFields.Values.Where(value => value.Field.Caption == NoSpamLabel).FirstOrDefault();
            if (nospamField == null || nospamField.ValuesAsString() != NoSpamValue)
                spam = true;

            Umbraco.Forms.Core.Services.RecordService rs = new Umbraco.Forms.Core.Services.RecordService(record);

            if (spam)
            {
                if (Action == "Delete Record")
                    rs.Delete();
            }
            else
            {
                rs.Approve();
            }

            rs.Dispose();

            return WorkflowExecutionStatus.Completed;
        }
    }
}