Paula, it sounds like a problem between your computer and yahoo. In that case changing aliases or yahoo accounts won't help. To get to a proxy server, use any of the anonymizing services available, the-cloak, anonymizer etc.. They usually have free, look only demo features. With an anonymous proxy, it is impossible for yahoo or anyone to id you easily. It is not impossible but usually takes a court order. That will at least narrow down the possibilities to
1. a problem between your computer and yahoo
2. It can't be you because yahoo cannot id from anonymous proxies.
I still seriously doubt that yahoo or anyone is blocking you. 1. Yahoo's influence on financial markets is widely regarded as negligible.
2. The amount of garbage vs info on yahoo is about 100 to 1. If they got rid of all pointless, lying, or just plain insane posts there would be very little left.
The only thing I've ever seen them get the least bit excited about is death threats. Unless you have been posting those, why would anyone bother with your account?