You cannot stop people to do illegal things and i guess bitcoin is just an isolated case. Any form of payment will do including cash, online payments or even in kind. Its such that, bitcoin has all the aspects to keep the identities of the person who is paying thats why its perfect for this.
If there's a way to stop criminals to buy illegal things through fiat, then it can be one of the best proposal that we can ever see to stop the usage of bitcoin in a illegal way. There's no way to stop these kind of transactions but the thing that we can do is to start with ourselves. Let us use bitcoin to the simplest and normal way of purchasing legal stuffs and just don't let yourself included to those shady transactions. Bitcoin is making our lives easier to send and receive payments quickly.