Would it be possible to close an oblivion gate from without?
If the gate itself is magical, could it be dispelled? Granted, it would take very powerful magic to do so, but what if the combined effort of a group of powerful mages came into play?

A Mage's Guild quest exploring an Oblivion Gate was direly needed I thought, and could easily have let us in on some much needed knowledge, instead of just one book.

Though other hyperagonal media may exist in theory, the only known transliminal artifact capable of sustained transpontine circumpenetration is the sigil stone.

Eh, I guess this answers your question: they more than likely exist, but as Hyamentar pointed out, there's such an absence of knowledge about the gates themselves that we can't even truly make an educated guess about it.

The only other piece of information I might know of related to this matter is the story of Battlespire and the gate that leads to the Battlespire itself. But I have a feeling that probably won't help.

Knight Mariel quoted from the jargon-laden http://www.imperial-library.info/obbooks/liminal_bridges.shtml

Don't know why an Altmer wrote it, though.

That increasing Altmer Daedra worship, perhaps? Or maybe the devs just did it for the hell of it?

Sure, a really fat telekinesis spell could help you remove the sigil stone from outside that gate. But i'm sure that's not what you meant.

High Elves have highly educated sorcerers with a strong curiosity to magic. It's all for science! sorcery.

Conjuration and liminal mechanics was Camilonwe's field of study.

okay, then I have another question. If more than one individual was to enter a gate and they became separted from one another, would removing the sigil stone by one party strand the other in Oblivion?

The guy in the cage in the Kvatch Gate didn't follow you out, did he? So yes, they would be stranded.

Never got how you were "cleansed" from the Oblivion "plane" anyway, just by taking the Sigil stone.

My guess is whatever runes inscribed on the sigil that notes destination. If you have it, it can take you back there.