a story of patriotism...

Besides, the official wasn't even in his right to instruct my dad to leave

Yes he was.
Thats his job, keeping order in meetings.

refusing to follow instructions of an official is not called "disturbing the peace" or anything.

Of course it is.

aaargh! please don't make me sum up every single article of the Dutch law! I've got other things to do than copy-translate-pasting it all onto subsim!

Its easy, the person claiming to be your father posted the writ on that blog you linked to. The specific crime is listed at the bottom.

"a 'prohibited place' is any place that's indicated as prohibited under the Law for Protection of State Secrets"

A prohibited place is any place covered by laws and by-laws....like a government building, though even a public park or road can be a prohibited place.

I fully agree on that one.
But:
1) I fail to see how anyone can possibly be annoyed by seeing a flag

You keep on going on about "the flag". It has nothing whatsoever to do with the flag, it is entirely about his conduct.

2) As I have said (and repeated until the end of ages it seems to me) it is completely legal to display any flags/banners and stuff. So even if anyone somehow felt annoyed by the flag, my dad was allowed to display it.

Again, it has nothing to do with the flag.