Can you pay via the dinerclub's website? When you set up the payment, you are sent a confirmation number showing date paid and amount paid - also, the check statement clearly shows the payment was made to diner's club so there should be no problem with later proving if a payment was made to the IRS or anyone else that might audit you. Additionally, I would think that the IRS is more concerned with the receipts that back up what is charged to the DC account and so long as your DC's bill reflects payments made with a corresponding entry on the check statement, I would think it should not be a real issue that you don't have a canceled check showing payment to the credit card company. I personally prefer this method of payment as the check statement says "DC payment" versus just a check number [when the online payments are made] but of course that is a personal preference thing I suppose.

However, I do agree that DC should let you opt out of it permanently and not on a month by month basis.