How serious is a cease and desist letter and can't i just stop using their brand after receiving it?

This would be an extremely bad idea. This would clearly constitute trademark infringement and worst of all you are doing it for a commercial purpose. In addition, since you are not licensing the character, you would be depriving Disney of royalties rightfully due them.

Lastly, there is no way to be sure Disney would not go immediately to court rather than send you the warning you envision. If you are making money off their trademark, you can almost certainly expect litigation.

Disney has always been very protective of their inventions, brands and characters.

I agree with Mr. Garner that your proposed approach is not a wise one.

Good luck to you.

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The C&D letter is simply the beginning. It is to inform you of the wrong you are doing and to warn you of further legal action. The fact you already know you are acting improperly will create "willful infringement" and that could really increase any damages you will be required to pay if you lose a lawsuit.

My advice is to approach Disney before using their property and ask if you can.

As the others have pointed out Disney is extremely protective of their intellectual property and would have no problem suing you. I hope you have a few tens of thousands of dollars that you have nothing better to do with then to spend on litigation that you cannot possibly win.