I'm sure this will be the first of many. 

Mute words can be bypassed by generating random text if needed
IP address blocking really doesn't solve it, proxies, vpn etc
Keys can be generated on demand

It's just a DOS in a way as it looks like regular content. 

Paid relays help but we still have freelays for now so as long as relays federate content, may be an issue due to replies.