I bet it would be very convenient, especially for those addicts with 50+ games in progress, to have a link (Message all opponents), which would create a PM to be sent to all opponents. So if the player has something to warn or tell all of his opponents, just click the link and send the PM.
Problem is, by the time you get someone on your ignore list, the damage has been done: they've already been able to spam you once. There will always be one more person willing to try it.
IMO, anything you have to say to 50+ people at a time, I don't need to hear about. If you're just not going to be around for a while, set the vacation flag and put your explanation in there; everybody knows what it means.
i've had 250+ games at times and usually keep a pretty high number going, so i think that this is a good idea. RHP can have a "spam standard" that says that "once you are identified as a spammer you will lose PM and forum posting privileges for a set period of time, e.g., first offense - 2 weeks, second offense - permanent loss of privileges. The only reason for a 1st warning is to allow for an honest misunderstanding that your notice that your cat just had a litter is spam.