Germany bans political opinions (being a Nazi is illegal there), so not sure why a European country would not be able to ban someone based on a religious opinion.
There's a reason for that: Adhering to Nazi ideologies is a dumb choice you make on your own and on top of that inherently evil.
Being one of 1.6bn worldwide Muslims is not.
I do not try to deny the fact that Islam has serious problems with radical ideologies and I don't think that conflicts with me being liberal. After all, the content of these is the same as those of far-right wingers or evangelicals that liberals despise as well.
However, as some of the more sensible posters have already pointed out, there seems to be no realistic solution to the current wave of terrorism (if you disregard the insane extermination fantasies of some of the racists on here). Deportation is just not legally possible with the majority of recent perpetrators being citizens of the attacked country. I do support the idea of cracking down more rigorously on radical preachers, locking them up in isolation or deporting them when possible.
Apart from the societal problems it's the practical nature of these new attacks. Those are no high profile attacks, no 9/11s, no transatlantic flight plots, no Madrid train bombings anymore.
While that means that they don't kill hundreds or thousands in one attack they are probably even better at getting people on the edge because even with the most modern surveillance technology there is practically no reason to prevent them anymore.
All it takes is for one of these brainwashed idiots to go downstairs, take a knife out of the drawer and head downtown. Social media and the internet does the rest.
Where this leads has been nicely illustrated last night in Turin where someone supposedly letting off a fire cracker and another one (or the same) yelling 'bomb' was enough to trigger a stampede that injured 1000 people...