Action is being taken, There is a reason multiple nations are fighting in Syria and Iraq, why America has special forces and troops stationed all over norther Africa and the middle east.

At home we are dealing with a few dozen people out of millions, who use simple hard to stop things like stealing a truck and driving it into people, in this case a backpack full of bits of metal and common mixtures of chemicals that can explode which you can probably get fairly easy,

What do you suggest?

Deportation will in fact kill more people then if nothing is done, you can't just deport millions of people without some of them fighting back, and when you're dealing with possibly a dozen terrorists over a few years and deporting millions of people even if somehow 99% of them go silently 1% is still more "terrorists" being created overnight then hundreds of years of the current situation, So that is completely out of the question

Banning immigrants? many attackers are people born in those countries, and at best you're preventing attacks in a few decades, and that is a huge "maybe"

The biggest thing that can be done is better integration, and better involvement within their communities and mosques, make it so people are willing to rout out those involved by rewarding them for it, there are plenty of more pro active ways that don't directly cause more harm to great numbers of people then if nothing is done at all.