I'm not saying I'm taking legal action if someone took my sprites. I just say I'd take action.
Spoiler: Actually, I have in the past (click to see content)
I helped Ironmugen to make/fix a special walk animation for his Krauser, I only touched the Krauser set because Iron asked me to, and he is my friend. Some guy took those sprites and turned Krauser into a cholo with a bandana. I respectfully went to the latino forum where this happened and requested the guy to use some other sprites, since I made those for my friend. He complied (or something along those lines, I don't remember).
Someone requested use of my classic stance for Scal's Robert KOF96~98 project, and I respectfully denied permission until Scal's Robert was released, because I made those sprites for Scal. User respected my request, even if he could have simply taken the GIF apart, and grabbed them from there (The GIF was my avatar IIRC) ... kinda like what Chucho did with CVSNB's sprites, and you see how that turned out even in this day and age.
You can ask, they can comply if they want. I can't stop them from actually using those, but I will at least take action by openly saying I disapprove of the usage of the sprites I made. That won't change a thing, but at least I'll have my peace.