Umm, not true after update 1.10. You can now "suspend" championship after single races (but be sure to start on the left race link (not the individual races to the right of it)).
There's a new button after you complete a championship race--the circle with the line thru it.
PS: the Veyron in the game is NOT "all that and a bag of chips". It corners like a brick on wheels. Straightaway/top speed good, racing not so much. Just didn't want you to get your hopes up.