Agreed. If someone has "clear ties" to a terror network, how come nothing can be done to stop such a person? Why the hell wouldn't they be on a no fly list? There should be measure that law enforcement and government can take to stop such a person. Communication with a terror group should be illegal, except in certain circumstances.
As far as other things that can be done, I think two things can be done at the community level:
One - we have to combat and speak out against racism and xenophobia. ISIS and groups like them love it when nations and people respond to such atrocities with suspicion and rage against entire groups of people, and with oppressive policies meant to ostracize whole groups based on the lunatic actions of a fringe few. Such attitudes and policies push alienated loners into fanaticism. It makes it easy for radicalism to lure them in, by creating an "us against them" mentality and giving these impotent losers a chance at feeling "power" no matter how despicable and fleeting that "power" might be, and it gives them vent for their petty little desires for revenge on a society they feel has rejected them. Note that many of these creeps are oddballs who are unable to maintain intimate or romantic relationships and are thus filled with rage, or who are socially inept and never able to fully integrate into any society. The lure of violent "warriors" who embrace them fully and appeal to their sense of "honor" and their fragile egos, is all the more great when the community promotes attitudes and laws meant to punish them not for their evil but for their nationality, religion, skin color or ethnic heritage, etc. So I firmly believe that in order to reduce the numbers of lunatics who embrace the insanity and evil of terror groups, we have to shower each other with love, acceptance and belonging. We have to show support to one another, talk with our neighbors, reach out to people who may be struggling or alone, stand up against bullying and bigotry. (This could help with mass shooter type situations as well).
Two - the religious Muslim community needs to develop a smarter attitude about this issue and communicate and educate themselves about what constitutes a red flag. For every religious community except the Amish, when a young person suddenly become super religious and quiet and studies the sacred text, after never having been that way, it is seen as a positive. Only the Amish seem to realize that that is not normal and can signify a real problem.
For Muslim communities, understanding that should be a priority.
When a young man suddenly becomes super conservative, quiet, ultra religious, secretive, his family and the community need to start asking questions, They need to find out where his head is at. What are his attitudes? They need to have frank conversations with young people like that, about radicalism and terror and how fanatics like ISIS are not of God, but are of the devil.
And if they suddenly get the urge to travel? That should merit some investigation and calls to whatever law enforcement agency is around in their country, to put them on national security radar.
