Is it possible to block access to certain IP address?

If for any reason you want to block the access to your form from some specific IP address, please, do the next:

In your form, go to the advanced options -> more options -> Form Pieces. Then create a custom Before Form. In the text box, please, write this:

if($_SERVER['REMOTE_ADDR'] == "123.123.123.123")
{
    header("Location: http://www.crosstec.org");
}

Change the 123.123.123.123 address to the address you want to block. When this IP address will come to your form, it will be redirect to  http://www.crosstec.org, or the URL what you want.