or troupe had a discussion about this recently and we feel that a ritual spell needs another ritual to dispell it. Although in so far as we can dredge from ALL the sources we're aware of, there is no RAW which explicitly states that.
However I would cite the whole paragraph in Cloudless sky returns and the overall feel of disenchant as what we in the Legal profession call persuaive authority (Not explicitly stating a rule, but providing good argument as to the spirit of the rule).
A