At risk of sounding all "elitist"....
The elephant in the room here is that very, very few of us are really qualified to make a judgement about effective security policies. It could be that "banning muslims" (to take a common example) or forcibly closing mosques would reduce our risk of terror attacks to 0. It's equally plausible that it wouldn't, because people could just pretend not to be Muslims and then undertake an armed struggle against a government that persecutes them. Either one is complete speculation by people who are frightened and bereaved. Understandable - satisfying, even, because it lets us pretend we know how to make ourselves safe. But if we're being really honest with ourselves, we know it's not really actionable.
Obviously, though, it would also be pretty unreasonable to expect everyone to just keep quiet after an event like yesterday. So if we stick to what we know, and try to avoid speculating on what we don't know, what are we left with?
Unless you're willing to personally join the security services who actually combat and try to pre-empt these people - and who know more than any of us do at any given time - then, unfortunately, all we can really do is accept that the world is a slightly more dangerous place than it used to be, and try to factor that in when we make daily decisions about our safety. All of us know that every time we walk outside we run the small risk of being caught up in an atrocity. (You agree with this even if you think you don't - if you believed that the likelihood of this was more than slim, you would never under any circumstances leave your house). The only "action" any of us can seriously take is to factor this small risk in to how we go about our lives and try to be as cautious as is reasonable.
I know that isn't exciting or satisfying, but I think that's about all we've got.