Woah, this thing is spinning out of control today.
First, it's a bit much to say FEMA Brass--suggesting senior leadership--instructed people not to visit Trump-signed properties. That's not what is being reported.
It's also completely unacceptable for a government agency to take that approach (unless they had credible threats of violence, but a Trump sign itself is not a credible threat of violence). Yes, there are credible threats of violence out there, but just putting a Trump sign in your yard doesn't turn you into a threat.
It also sounds like FEMA brass did the right thing: they fired the person responsible. There are stupid people--and people who make stupid decisions--in every large organization. You can't completely prevent bad actors. And--to be clear--that kind of instruction is a very bad act.