The whole point of the carnet is that you don't sell a vehicle in the country which it has been temporarily imported into.
You might be able to bribe a customs officer into stamping it out but I am not sure what the consquences of getting caught are, very serious in Iran I expect.
Apart from that you are showing a total lack of respect for the laws of that country and you will also be creating problems for everybody else who uses them by undermining the system, countries have been known to suspend accepting carnets.
To put it bluntly don't even think about it.
If gaffer tape doesn't fix it then you haven't used enough tape