I had a recent experience with DCC while traveling in Ireland. The charges were low (less than $10) and Chase covered the difference between the DCC rate and Visa's rate. None of the representatives or the dispute department knew or understood what DCC was or how it worked.

I would call the dispute department again, explain DCC, and state that you'd like to dispute a partial amount. Look at the date the charge posted, look up Visa's exchange rate on that date, and use your receipt (which I hope you kept) to calculate the difference of DCC's bogus rates to see how much you should dispute.

I hate DCC. Next time, use an American Express card, like the Platinum, as Amex doesn't allow DCC on its network. As you hand your card over, always ask to be charged in local currency and don't sign if you notice that DCC was used.