Not naive at all, APG. You could institute a blanket rule that all political messages of any kind whatsoever, even the "uncontroversial" ones, are banned at least in the immediate environment of the race. I'm not sure what's controversial about asking for these particular policemen to be arrested and charged, given the circumstances, but there we are.
The alternative is to allow people to spread whatever message they choose, as long as they're happy to accept the reaction.