I actually thought of a better plan than my previous comment:

• Whoever decides to enter the streets/track area during an active race is fined proportional to the number of competitors with the money going into the competitor's medical fees/health insurance, next event's prize pool, local charity/nonprofit organization.

Easy to track:

• Cameras everywhere by spectators, I'd imagine it's impossible to escape perception at an event like this.

Easy to enforce:

• most if not all spectators would probably agree with the idea.
• good luck fending off a classaction lawsuit from the competitors, event hosters, and maybe even some spectators for ruining the event.