Redirect connection to another IP Address

Hi, I've still got this spammer trying to use my machine to send Spam. My ISP (NTLWorld) seem either over worked, or isn't going to take any action. So how can I redirect any connections to port 25 from a specific ip address to connect to another ip address. Basically I would like to redirect all the connections back to the spammers machine if possible. Is this possible, or could this cause more problems. Regards Adam