For example, I have a free Yahoo account, but I had to join with my regular email user name.
True. It's been a a while since I signed up for anything like yahoo. I have had yahoo for a while now.
The problem is that there are unlimited web based email accounts available. Also unlimited ISP addresses available too.
An email account can be banned and so can an ISP address but the trolls just get a new one and join again.
To trace these a-holes would require a court order from a judge and a judge is not likely to issue one unless there are threats of specific violence. Not likely to issue that court order because someone is being obnoxious.