When I was thinking about ways to solve the problem, I didn't think it was a big deal in terms of balance. I thought the arena would be less fun if people just used the same rainbow deck + the oracle's cards.
But, now that CCCB outlined just how easy it is to to that, I get the feeling more people are going to switch to his oracleBow deck idea, and make the arena even more monotonous.
In my opinion, it's a bad idea to just ban cards that encourage the oracleBow deck idea, and instead encourage their use some other way, like making the oracle's cards not count towards your deck's limits. In other words, with the 5 copies of the card the oracle uses, plus the 6 you can use, you are allowed 11 copies of that card in your deck. This allows new decks that wouldn't be possible with standard rules, and therefore a neat challenge for skilled deckmakers.
OR, in reference to the poll you created, crutch decks by requiring them to include 5 of some specific quanta source for the element of the card they got from the oracle. So the only sources could be mark, pillars(except qualtum pillar), pendulums, ability (devour, etc...) , or the likes of soulcatcher, solar buckler, etc.
I think that just banning rainbows altogether is a bad idea. I have seen a few good decks that needed rainbow pillars or supernovas that didn't just clone the oracleBow. A mass mindgate deck, mass improve deck, and even a clever rainbow poison deck that I'm afraid to leak out many more details of :p Rainbow quanta sources are a necessary addition to quite a few good decks. And telling these players that their deck isn't allowed isn't fair to them.