Can checkpoint flags be turned off?

Based on what I've read here:

As long as you keep within the checkpoints, it’s fair game.

It sounds like the checkpoints are a core mechanic and not able to be turned off. This is actually the source of a complaint in this review (cached version, original link is actually dead):

the checkpoint placement can be a little frustrating at points – where you’ll narrowly miss a checkpoint banner by inches and then be forced to restart the race – which breaks the sheer pace and momentum that Horizon champions everywhere else in the game.

Actually you're not forced to restart the race, your car position is just reset to a place before the missed checkpoint and you just go from there, without stopping the race.