function ModalShow(){
// When the user checks the checkbox...
localStorage.setItem('checkbox', 'true');

// When a user visits the page...
var checkbox = localStorage.getItem('checkbox');
    if ('checkbox'.checked == 'true') {
        $('#myModal').modal('hide')
    } else {
        $('#myModal').modal('show')
    }

}

// html
    <div class="modal-footer">
                  <p>usual terms and conditions acceptance blah.</p><input id="checkbox" type="checkbox" class="agreement" required>
                  <input type="submit" class="modal-button btn" data-dismiss="modal" value="Submit">
                </div>
              </div>