When we look at the problems we face with IS fighters in Denmark, we need to realize that we are in a war with them. Not just in Iraq and Syria, but also in Denmark. We try to be delusional about it, but when the IS supporters believe that the war also encompasses Denmark, and includes muslims in Denmark, the muslims in Denmark that supports IS are enemies.
How do we then treat these IS supporters that, often openly, show their allegiance? First of all, we should not fall to the same depths as they do, that would be wrong and inhumane. We need to use the tools that is as humane as possible.
This is why I have suggested the expatria tool. If you support IS, you will be excempt from your allegiance to the democratic country you live in. You cannoth both support a caliphate and a democracy.
This will of cause be in contrast to human rights.
Since human rights are basically a version of Athenian democratic thought, a good starting point would then be to go back to the original thinkers, and hear about their opinion.
According to Aristotle, there are two prerequisites one has to fulfill to be a citizen in a democratic city (they had cities at that time, not nations). One is to be a part of the legislative process (obey the law). The other is to be a part of the democratic process (vote and take part in the discussions that lead to new laws). If not, you are not entitled to a passport.
Since the islamists fulfill neither, it makes sense to take away their citizenship. In fact you could say, that the defense of the democratic state needs to take this defense in regard to survive. Otherwise the Islamists could vote themself in, and change the constitution as the Nazis did in the last century.
So, we need the police to get started. It should not be so difficult, most islamist are quite open with their affiliation on both Facebook and Twitter, so they are easy to find.
G-d bless the will to survive as a democratic state.