Shouts should have a 30s cooldown, minimum. I know people love to epeen over public channels, but in the context of answering questions and being helpful, sending the asker a tell can do the trick as well as open the door for clarification. This has the side effect of curbing troll chatter a bit, too. Otherwise, I could see a see a system where if you get X amount of blacklists, it tiers you up into cooldowns. 10 people = 10s, 20 = 30s, 30 = 1m, 40 = 5m, 50 = chat lock and auto-GM report. If it's found by the GM that the message isn't spam and the blacklisters are all in the same FC trying to troll someone, then they could get locked out instead for abusing the system.