No, burning a seal is not disrespectful to a demon. It is a time honoured method of closing off the connection, and one EA recommends in all of his books. The only people who seem to think it is disrespectful in some way are demonolators like S. Connolly, who view demons through a religious lens.
The idea that it is “disrespectful” comes from traditional Solomonic magick, where the demons are threatened with fire and torture if they don’t comply with the demands of the magician. However, it is the intention when burning the seal that matters, not the act itself. I burn my seals all the time once the results have come in, and have never had a problem. As I burn it, I thank the spirit for the work they have done on my behalf.
I have never heard of any spirit but the one called coming from a demon’s seal. The seal is a representation of that specific Power. In fact, one of the ways to discern an impostor spirit from the real deal is to ask them to state their name, or to display their seal.