After going through a long list of completely impossible steps, we are left with options such as:

1. Reducing immigration from nations with a high level of religious intolerance (which functionally means Salafi Islam)

2. Making residency and naturalization conditional on a commitment to Western values so that one can be deported for, say, marching with signs that say "To Hell with Democracy" - while this limits free speech, it doesn't limit the expression per se, only recognizes that there was a condition to admission and, if broken, such admission can be withdrawn

3. Openly banning the spread of Salafi Islam to the full extent we can make that legal

4. Imposing criminal penalties for any internet services that knowingly permit Salafi literature to be propagated

5. Taking military action to destroy training camps, informational centers and other gatherings of jihadis

The thing is that we're already doing most of this. The big failure has traditionally been Saudi Arabia, the UAE and Qatar, who have permitted funding. Let's see what happens with the new center announced when President Trump went there. I have heard from people who have met with the new king of Saudi Arabia that the Royal Family has recognized the danger to itself as well as its kingdom from the extremists and is finally doing something about it. It will take time, though.