Yeah and felons had due process. That's how they were convicted. A collateral consequence of their criminality (the determination of which included due process) is being disqualified from owning firearms.
As long as you have due process protections (you know, where you actually have a hearing, have an opportunity to put on evidence, etc.) before you strip somebody of their rights, then I doubt any person has problems with taking firearms away from the mentally ill. In fact, if you are adjudicated mentally ill, you are disqualified from owning firearms, just like with being a felon. And you can always go through the process of adjudicating somebody mentally ill. What you cannot do is simply take somebody's guns because you don't like them. What if they're on the no fly list? What if the sheriff thinks they are nuts? If they can take a person's guns because he's allegedly dangerous, then why can't they just throw him in a cell to keep him - and us - safe?