Howdy!
Ping muting is really interesting. One one hand, they're the "last line" of communication in League and cutting all comms (as suggested below) is worrying; a team game with no communication at all is a really bizarre concept. On the other hand, getting ?????'d or !!!!!!!!'d as much as they possibly can is annoying af.
Ping rate is already throttled. It's actually an ammo-like system on the back end that lets you build up pings over time and spend them. Having differential throttling based on mute status is a little complex because the pinging-player wouldn't be changed, just what other players see (because it's likely that some players haven't muted them). That said, it's certainly not unsolvable by any means.
Not sure that it's the solution to ping-spam, it may or may not be, but the problem is certainly worth a look again (we've talked about it before). No promises on when we'll get to it, but it's good to be reminded. It's weird, when I play and have this happen it just pisses me off...somehow I haven't re-raised the issue as a designer recently.
Thanks!