I have a form that contains information fields for potential customers. Spammers are entering web addresses in the address field. I want to have an error message when the field contains "http://"

Here is the form code:

```
<label>First Name:</label> <input id="first_name" name="first_name" type="text" size="20" />
<label>Last Name:</label> <input id="last_name" name="last_name" type="text" size="20" />
<label>Address:</label> <input name="address" type="text" size="30" />
<label>City, State &nbsp;Zip:</label> <input name="city" type="text" size="20" value="City, State Zip"/>
<label>Phone Number:</label> <input name="phone" type="text" size="20" />
<label>Email:</label> <input id="email" name="email" type="text" size="30" />

```

Here is the error code I have:

```
    function validateForm(){
    message = '';
    error = 0;

    if (document.contact_form.first_name.value == '') { 
        message = 'First name is a required field\n'; 
        error = 1;
    }
    if (document.contact_form.last_name.value == '') { 
        message = message + 'Last name is a required field\n'; 
        error = 1;
    }
    if (document.contact_form.phone.value == '') { 
        message = message + 'Phone Number is a required field\n'; 
        error = 1;
    }
    if (document.contact_form.email.value == '') { 
        message = message + 'Email is a required field\n'; 
        error = 1;
    }   
    if (WHAT GOES HERE TO SHOW THAT THE FIELD CAN'T CONTAIN ANY VARIATION OF 'http://?') { 
        message = message + 'That is not a valid address\n'; 
        error = 1;
    }

    if (error) {
        alert(message);
        return false;
    } else {
        return true;
    }
}

```