There's absolutely nothing that can be done to prevent this.
It doesn't matter whether it's cash, bitcoin, or STEEM. It can be laundered in some form.
Some Bitcoin mixers would exchange your coins for coins from freshly mined blocks at a fee. That's not traceable unless the mixer is ran by the feds.
If anything, Steemit is a lot harder to launder money on, since it's directly linked to your account, on which you'll struggle to build any sort-of reputation or STEEM without people knowing who you are in some form.
I mean everyone would see me transfer coins to @poloniex or @bittrex, if there was some sort-of steemit mixer, it'd be pretty obvious after a while when they start receiving tons of MEMOs.