While it's not preventable from them reuploading your work, there's ways to make it obvious they aren't the original content makers. Water Mark your skins- open the skins and edit them on a 2D layout and use the invisible parts of the texture to add your own secret flair. Many skin making noobs won't even know this exists. I recommend uploading the skins on all major skin sharing platforms under the same username, NameMC, Planet Minecraft, and MinecraftSkins.net for example.

theres not a way to stop them but you can always report for stealing your skin

Theres probably no way to stop them unless you put some hidden water mark in your skin or dont upload it.
Because people can freely download your skin, people can much as easily put it up online and claim it as their own