That is such a difficult question to answer, I'm not sure really .............would depend upon the individual concerned I suppose.
I would hope that I would be able to have some influence upon them and work with them to discourage any further buying. They maybe need some guidance and educating!
Rescues are there though to pick up the pieces of other people's mistakes, maybe it's not for me to question how other people lead their lives but just to be there for the animals when they are in need, for me the animals are the most important part of the equation, they always come first.
I have had a similar problem with cat rescue, over a period of a year I took 6 cats in total off an unsuitable owner and each time she got another. Eventually I had to call it a day for my own sanity, I realised it would be like trying to stop the tide coming in, so I had to walk away. It's not something I did lightly but I know I did the right thing even though it felt wrong.
Are you having that problem at the moment can I ask?