Stop SPAM from custom form

This form is receiving a lot of spams and I would like some advice on what I can do to prevent it.

I should not use any plugins nor any Captcha.

 function innerpages_form($atts, $content = null)
{
    session_start();
    // Attributes
    extract(
        shortcode_atts(
            array(
                'title' = '',
            ),
            $atts
        )
    );
    // Code
    //global $wp_session;

    return ' div class="sell-fast-form"


    form action="/sell-now/" method="post" id="myForm"

    h2 style="color:#f15a29"Need To Sell Your House Fast?/h2br

      h2Property Address/h2
                input type="text" class="Property_Address" name="Property_Address" id="addr" placeholder="Property Address" value="" required 

                h2Phone/h2
                input type="text" class="Phone" name="Phone" placeholder="Phone" id="Phone" pattern="^[(]{0,1}[0-9]{3}[)]{0,1}[-\s\.]{0,1}[0-9]{3}[-\s\.]{0,1}[0-9]{4}$" required title="xxx-xxx-xxxx" 
                h2Email/h2
                input type="Email"  class="Email" name="Email"  placeholder="Email" id="Email" value="" required 
                input style="display:none;" type="text" name="robots" value="" /
                input type="submit" class="click_here" onsubmit="return myFunction();" value="nbsp; nbsp; nbsp; nbsp; GET MY FAIR CASH OFFER nbsp; gt;gt;gt;"/





  script
        function myFunction() {
            document.getElementById("myForm").submit();
            return false;
        }
        /script
/form

        /div

      ';
}
add_shortcode('innerpages_form', 'innerpages_form');