The governments of Europe no longer hold a monopoly of violence. Terror groups and MENAPT groups have brought a diverse range of violent threats to people.
Building a robot army to solve this is one viable solution that is hardware and software based.
The efficient & practical way is to stop renewing residence and work visas, making acquisition of visas harder through complex demands of high language proficiency, and a high amount of money as proof-of-funds, perhaps requiring a local referral too. It's what countries that don't want immigrants but don't want to say it out loud do.