Hi BNMedia, I would say leave them to it. What does it matter to you that they are using your site name as a subdomain? As long as they aren't bringing your brand into disrepute I don't see the harm. It looks like the subdomain you are concerned about is just a blog promoting your site anyway so let them carry on - the more people out there promoting your brand the better.
I would imagine if you contacted them and asked them to stop using the name kinkykicks they would reply along the lines of "OK, we won't use it all" and will stop promoting you. If someone types in Kinky Kicks into Google you are the top three spots anyway and it's not until fourth spot that the subdomain comes along - so I think it's unlikely people would sign up through them rather than you directly - but if they do, it's better to catch them through an affiliate than lose the sale altogether.
I like your site by the way - hit me up on ICQ 23325595 if you want to do some cross promotion sometime
