Add a basic reCAPTCHA form to php registration page. Can be any captcha, must integrate with existing php signup. Can be done with javascript or php, whatever is easiest to prevent software from submitting listings. Can provide the URL, source code, whatever is needed to get it done easy and fast.
I can implement recaptcha on your form, easily and quickly. I'm online for at least 5 hours so if there is any problem, it will be fixed right away. You can check my reviews to see that I'm a fast coder. I can start right now.