I don’t see this as any different than the cops seizing your money or other assets on a suspicion. In many cases you don’t get any due process at all.
So far, the USSC seems to have smiled upon this. It also seems reasonable that if your neighbor is acting crazy and brandishing a gun, it should be taken away from him.
Hell, if your significant other files a protection order against you, they take your guns...
You are right. If someone threatens to shoot up a school the police should arrest and disarm them. Due process comes after, just like Trump said.