When nothing else works, get offended. Indignation, if you keep calm, can be useful in lack of better arguments. You look stiff but dignified. You may get away with it.
This reminds me of a story:
Reposing peacefully under his porch, Nasreddin sipped warm sweet tea, with delight. Hassan, the neighbour, chose this moment to come and ask for a small favour.
“Please Hoça, lend me your donkey. I need to carry my wood for this winter. I’ll bring it back, healthy and well fed, after tomorrow.”
“I am so sorry," replied Hodja, "unfortunately I already lent him to my mother in law. He's away for a week to carry her water.”
As Nasreddin finished saying this, the darn beast could find nothing better to do than to bray loudly from the stable.
Hassan exclaimed: “Aaa! As I hear, Hoça, the animal is denying in his own voice what you just said.”
At this Hodja turned red with anger and yelled:
“Away with you, boor! If you take my ass's word over mine, we have no business talking to each other! Go and ask the donkey to lend me!"