Holding Companies advertised responsible for spam

lunatick writes: "I have been getting a ton of spam on my forums. Despite e-mail activation, image text input, auto-banning anyone that trys to input a webpage I get an average of 20 new sign ups a day and 2 actually activate and post Spam. I added a $1000 per day advertisement fee for the forums as a TOS, but how do i enforce this. I tried billing 1 company and was slapped with a harassment threat. I obtained the address through a whois lookup. Does anyone in the /. community know of any legal president for holding the advertised companies/websites liable for the spam of the advertising companies they hire?"