The whole dispelling a Magic Circle is already provided for in the ritual description. Anyone with a level = caster's arcana check - 10 breaks the circle automatically if they try to pass. So a tarrasque can break a circle cast with a check result of up to 40. Its possible to hit that result on a good die roll at around level 10, maybe even level 5 if you superoptimize arcana, but only on a roll of 20. To have a 50/50 chance of setting up a circle that would hold a tarrasque would require a superoptimized arcana character of at least level 20. Even a level 30 character can't achieve a 100% certainty of setting up such a circle, though it wouldn't be too difficult to do it with assistance.
As for the specific situation of the berbalang, the duplicates have the same keywords as the original and thus won't be any more able to break the circle than the original. Depending on how you read "affect the boundary in any way" a creature inside might be able to create an effect which would indirectly affect the circle, but its going to be up to the DM to decide exactly how indirect the causative act must be in order to get around that limitation. Since the limitation on affecting creatures on the other side of the circle also applies to allies even if you rule that the duplicates can be conjured outside the circle there are limits on what the original can do with them since they cannot affect each other.
I don't think I would allow the Dispel Magic utility spell to affect rituals in general. I think you would need a ritual version of some sort to have that kind of effect. I can't find anything listed by the compendium which has been published in
4e that does it. I'd suggest it as being a level 1 ritual since it seems a fairly basic aspect of magic. A simple opposed check vs the ritual caster's check when the ritual was set up should work fine. It might also work to counter other static magical effects of similar nature, possibly only temporarily depending on what they are and who created them. So for example divine level magic probably can't be removed permanently by a mortal ritual unless certain conditions are met, but might be neutralized for a time.
All of this kind of stuff is really relegated to the fairly abstract provenance of the DM in
4e and should mostly fall under use of the arcana (or maybe religion, heal, or nature) skill and possibly specialized rituals or items. Personally I think the overspecification of free-form magical effects in earlier editions was a bad idea. It tended to lock DMs into specific hard and fast rules about different interactions that were usually not appropriate to the specific situation and meant either the DM had to resort to "well, this guy's spell is special" kinds of excuses or else you could end up with plot busters.
4e puts it much more firmly in the DMs hands so that you have more ways to make things fun and interesting without players feeling like you took away options they should have.
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