maybe a yellow card? Two yellows = penalty or whatever?
That's what they do in cross-country skiing. It's very subjective and sometimes almost random what gets called and doesn't get called, and as usual is open to calls of corruption when particular teams and athletes seem to get away with murder. However, it is also very clear; athletes on startlists, in intermediate sprint results and on results pages appear with the yellow card clearly displayed next to their name, and if they receive one mid-stage it is added almost instantly via the transponder data.
That might have helped explain some of the furore in 2010 and 2017, if it had been
clear that Renshaw and Sagan were already on a yellow card from previous incidents. Although Renshaw's were so close in time to be consecutive, but it would have helped immeasurably in Sagan's DQ to at least understand the thinking behind the decision, because the comments about his actions at the intermediate sprint weren't public until after he'd already been relegated for the irregular final sprint; had he appeared with a yellow card icon in the intermediate and stage finish results, then people might have known he was already on warning and been less taken aback when the commissaires took the nuclear option.