URL Redirect Page and spamers

Hello!

I have a site on an old EE 1.7.3. I noticed that in search engines there appeared a lot of pages like http://www.mysite.com/?URL=http://www.spamersite.com (To proceed to the URL you have requested, click the link below)

How can I get rid of this? Is it possible to simply remove the functionality of this page with a redirect (then there will be just 404)? Or advise how to display 404 when accessing to url http://www.mysite.com/?URL*

Thank you!