It's really bullshit for someone to completely try to steal your art and name like that. They're probably just jealous that they don't have an art style and characters as adorable as yours. But I am a little curious has to how you use artwork to get cybersex.
there are sad and desperate people like this everywhere. however, i wouldn't worry about it unless they are defaming your name or trying to sell your work. if you feel like they are doing either of those things, i would try to address the matter personally first. then, if you feel like you are helpless, to ask for advice without giving out a name. and only as a last resort, pointing out the person to all your fans.
i only say this because some people can be needlessly cruel and vicious to others who are taking something away from someone they might be a fan of or even idolize, and it might indirectly make you look bad - even if it wasn't your fault.
don't get me wrong, art theft is a serious matter. but i think most are caught and face the consequences of it, even without the original artist ever being involved or even knowing about it.