Wrong billing + bad attitude

I made a booking for a group at Mercure Randburg, South Africa and gave specific instructions that any extras should be charged to the respective rooms and all money collected when guests are checking out. The guests requested transport and paid it directly to the cab driver. When they had left the reservations lady gave me a bill that included R750 for the transport. I queried it and asked the remaining guests who also confirmed payment was given to the cab drivers. Now they keep calling me and harassing me. I explained that besides them making a claim that all guests are confirming is not true, my instruction was clear and I dont know why they are harassing me. The ladies SHOUT & insult you and I said to one of them, you like arguing and I dont, so I asked to meet their GM with everyone involved and they are refusing. Please assist to resolve this