Europe either has to create an intelligence force similar to Interpol which is happy for each country to talk to another or the borders close.
The politicians have to decide what is best for their people, they have to reflect on what has happened not only in the last twelve months, which has been horrific, but for the whole millennium. The ideological view can no longer be supported a pragmatic view must now be adopted.
Once the borders are closed more action can take place for instance not letting people back that have travelled to Syria and surrounding countries and have taken part in the conflict there, expelling people who have expressed views that could be seen as incitement.
Most of the actions that need and could take place require either of the first two options to be implemented.