Is there a way I can hide my Steam account or something?

You can make your profile private, and that would do it.
Otherwise, find a way to prove beyond a doubt that it is having a serious negative impact on your life, and then sue them for libel.

"I'M NOT GIVING AWAY GAMES,IT'S A FUCKING TROLL ON 4CHAN POSTING MY ACCOUNT."

And people keep adding you... Geez, so many analphabets -.-'
Try changing your custom ID, it might help you.

Not aware of any way to do so, but maybe you can leave a note on your profile stating that what they read on 4chan was not true.

Granted, it won't really help since most people stop reading after seeing FREE, but its worth a try.

Perhaps a change of your custom URL if that is what they have used. If not, maybe Steam Support can help, if you can wait for them.

Privacy settings would be my best guess.