watermarks and art thieves

sorry that my recent artworks have big watermarks. I know that they kill the pictures, but I don't feel like I have any choice. last days I recieved a note, that someone took my adoptable characters and sell them at other website. that liar robbed not only me, by selling my designs, but also people who bought these designs (both my customers and people who bought faked adopts from that dishonest person). I feel really upset, I even don't know what can I don, at least it's the Internet.

first I was like "I'm gonna delate my account here, no more publishing my pictures" but hah, silly me, it would be useless - my pictures are already everywhere around the tumblr/pinterest/weheartit. and I'm not trying to fight with that (untill someone claim my picts as it's own). also.. drawing is no fun when you can't share it :c I would terribly miss all of you! even when I don't reply for all your comments it a big pleasure and fun for me to read them.

I thought that small signature "fuki" was enough. but lot of people started to cropp the pictures to remove signature (once one girl at instagram cropped some picts with my Offelia and sent them to one model with notes "hey I made a fanart for you". wtf, really?). I know that my recent watermarks also are not impossible to remove but maybe it will take some more time _{(if someone wants to steal my artworks then he/she needs to try harder, hah! XD )}

how to deal with art thieves? or with people who trace your artworks or only change some colors and pretend that they're the authors? or steals your OCs and fake people that "hey that's me"
any ideas? :c