'Let us not forget that violence does not and cannot flourish by itself; it is inevitably intertwined with lying. Between them there is the closest, the most profound and natural bond: nothing screens violence except lies, and the only way lies can hold out is by violence. Whoever has announced violence as his method must inevitably choose lies as his principle....' -- Alexander Solzhenitsyn
If you want to make an extremist more extreme just ignore him. He'll feel like he has to take more extreme measures to get through to you.
If you want to influence how an extremist thinks, talk with him. Reflect back to him what he is saying and ask penetrating questions. Yes it's difficult, but what options do we have on the individual level?
We hear a lot of condemnation of extremism, but what exactly is an extremist?
Here is my definition of an extremist and how governments should address the issue. Read on