They are ripping me off!

I rented two automobiles in June, 2014 with Dollar/Thrifty at the Munich Airport. The attendance gave me the option of returning the car full of gas or prepaying...I told her that we would return the cars full, which after paying $154 for diesel, they were returned FULL. After receiving my VISA statement, I noticed they charged me and additional $170 for diesel. I emailed them about the problem, sent them receipts, and they continue to refuse to reimburse me for the unauthorized charge to my VISA account. As a result, I rented two cars for three days and it cost me over $1000! Beware of renting from this company. They will RIP YOU OFF and blame you for the problem!