Complain directly to the General Manager.
If they are not able to fix it send a letter to the hotel owner and cc the Tourism Board or similar body in the country explaining the practice which should be illegal.
People are being tricked into signing something fraudulent. There is no "convenience" in DCC.
I have advised two hotels that I will no longer stay with them as a result of this practice and in both cases I have followed up with a letter to Tourism and the Ministry of Industry.
This probably sounds like going overboard- however the practice is so low that there is only one way to deal with it.