The economic system is successful because in general there is no way to stop someone from doing something economic. Much like how you can pull with a piece of string but not push; in economics you can take action but it is very challenging to prevent an economic action from happening. It is possible to do in the small but devilishly hard in the large (c.f. the content industries trying to deal with piracy for a good example).

Divesting just shifts profits away from ethical people towards unethical people. It is a bad strategy.

And the banks know that. If they stop investing in something it is because an actuary somewhere has worked out that it is risky and unprofitable.