They go through a judge at some point after they are executed. There is no due process before the person's rights are stripped away.

Sure, there's a solid argument that red flag laws have a 1) a lot more due process than no fly lists and 2) enough due process to make them constitutional. As I said, I don't know the details of the different red flag laws, and haven't formed an opinion on that.

But the comment I was replying to made a very different and I think highly indefensible argument: That the difference between red flag laws and no fly lists was the former was more effective.

If the individual isn't involved, it isn't "due process".

Warrants aren't due process?

It's pretty clearly the case that a process where the individual is not involved up front but retains the ability to challenge an unfavorable outcome can, for some things, be the process that is Constitutionally due. It's less clear exactly what the boundaries are for that, because the Constitution doesn't really explicit provide rules for determining what process is due for any particular action, leaving it in practice for courts to evolve standards.