To me, the
Valar were not completely unjustified, but neither were they justified in the matter. I chose "No" in the poll because my opinion seems to lean more towards it.
It is hard for me to explain things in good paragraphs, usually, so to avoid confusing you all, I shall just make lists of them. (Sorry. But I am never praised for eloquence anyway.

)
Justified because:
1. The
Noldor were sneaky. They rebelled firstly in silence, then just broke out, like the eruption of an unsuspected volcano. They followed
Melkor, although it was not Melkor that had given them sanctuary. The Valar must feel hurt and betrayed, and thus sought to teach them a lesson.
2. A third of them (Maybe?) killed the Teleri. The fencing might be for those Thirdlings, for them to recover their equilibrium and start again. Without the ban, some of the more arrogant and spiteful Noldor could return just to harass them and then go back to
Middle-earth again.
Unjustified because:
1. Like
Turgon (I think? And perhaps someone else too) said, the Valar did not only fence out the Noldor, but also the other people of Middle-earth, including the yet-unborn
Men.
2. The shadowy seas were a bit too much too. Melkor would not dare cross the open sea.
Ulmo and his people lurked there. He might retry the Grinding Ice, so why had the Valar not thought of this?
3. Losses on Elven lives should have been able to be prevented. Could the enchantments not just turn them around back to Middle-earth instead of drowning them in pain and misery? And one ship drowned just in sight of that land, too.
4. They blockaded all communication from and to Middle-earth. Not even
Melian, who was herself a
Maia, could contact her brethren in
Aman. So how were about the other
Maiar that lived there side by side with the younger Children? Why did those who lived in relative bliss in Aman also shut them out?
I know, it all did not only focus on fencing the Noldor out, but many matters branched out from that single thing.
The
Ainur, despite their unearthliness, were capable of 'earthy' feelings to some degree, and I rather view them as children learning the things together with
Elves and Men and
Dwarves. And as children, they also 'grew up' - mentally and emotionally. I just wish that the cost were not so high. (Oh, and, apparently, they had that possessive trait we had, too. They raised the Pelori and all to prevent Melkor from going back and destroying their home - again. It is one of the reason of that big defense system... What would you make of that, eh?)
Turgon: Sometimes someone must learn a lesson before he is forgiven. But I would agree that a lesson as painful as losses of life in seven
ships was too much.