I would do two things. First look in your attachments directory and see if it is there. Do you have ssh or telnet access to the machine? If so try and find file sizes of folders and see which one is to big. If it is *nix do ls -la which should show you everything in a directory. If it is windows make sure you can see hidden files. Or download your entire site via ftp and look for it locally then try to delete is over ftp. This would be a short term fix. I would then contact your host, actuall I would contact them anyway, explain what happened and see if they can help. It may be worth reinstalling your site to try and get rid of this log file in the future. Your host may also be able to help pindown both the location of the log file and what is creating it. Also, if you have the user's ip address, goto this site
http://www.arin.net and look up their isp by their ip address. Arin is like whois but for ip addresses. Then contact their isp. Have fun with it, make the user's life crap if you can, get them kicked off of their isp. I may seem mean but I got hacked and my server crappity smacked up from a guy in New York and I know how much it sucks.