I think that Yahoo should be held accountable. When you do a netstat, it is a yahoo IP address booting you. So the Yahoo service doesnt protect its customers from booters, and worse, the Yahoo service locks up your machine and prevents from accessing other services while you are being booted.
The only way to stop this is to force yahoo to act, that means holding them financially responsible for the harm their poorly managed services inflict on people. If they cant manage messenger responsibily, it should be shut down and make room for a competitor to replace them.