how can I reprograme my cars key (renault laguna)?

i need help to know how I can reprograme my car key .I bough from ebay and I had no knowledge that it's impossible to reprogram one key that belong to another car renault to my car. to me was all the same and just need to reprogram again . but when I took the new key there they told me that impossible because the keys are coded with chassis number within and if I tried to put a new code the key won't accept because I need to put my chassis car number in the key and the key already exist the old chassis number that still in the key
if any one can help me how to do it and where to take
please
thankyou