Usually when I see a burnt disc, that tells me one of 2 things.

1. The person does not know how to properly use a clutch and has fried the disc.
or
2. The clutch no longer can hold the power its given and slips.

Since youve taken it to them before, they dont have a record of that? Or you never got one to cover your ass incase something like this happens? If you have proof of it, and not just someones words, tell them to give you the answer, the clutch is the problem and has been for some time.