I'll try to find a copy of it, but if you search back on my old links to June of 2007, I think that's when I gave my wife the financial "I have decided that I will no longer pay for you to conduct your affair" letter. I stopped paying for her tummy tuck payments, lingerie, hair coloring (still paid for basic cuts) -- ANYTHING she was doing to make herself more attractive for someone other than her husband.
You need to do this very calmly, and not from a place of spite or punishment, but rather from a place of "Why would I use family finances to help you conduct an affair that could destroy our family?"