Is it ethical for PhD supervisor to forbid publication in a given topic Six months after my PhD graduation, I submitted a paper to an SCI journal as a sole author. Apparently, my PhD supervisor is the technical editor for this journal. He wasn't pleased that I submitted a sole-author paper without informing him. 
I formulated the idea and wrote everything from scratch. However, he claimed that's a generalization of what he's currently working on. After I proved to him that they aren't related he went ahead to forbid me from publishing in that area as it's likely to interfere with his. That's after he made me divulge all my on-going investigations. He said it's better we are collaborators than competitors. I didn't set out to compete with anyone. Most ideas I gave him during my PhD were suppressed only to be given to other students two years later. He made me the last author in those anyway.

Question: 
1) Does he have the power to do this?
2) Being that he sits on the board of top SCI journals in my field, how do I overcome this?
Your former supervisor's behaviour sounds entirely unethical. I'm sorry to hear that he is unnecessarily causing you distress. So, to answer the title question: No, it is not ethical to forbid anyone from publishing in a given topic. 

The justification given -- that it is better to collaborate than compete -- is entirely beside the point. Yes, it is good to collaborate. However, once the work is complete, it should be published and the authors should be those who contributed. 

It is understandable that your former supervisor would have liked to be informed. Everyone wants to be kept in the loop. But he has no automatic "right" to be informed of your activities after you graduate. (I'm assuming none of this work happened while you were his student.) Furthermore, there is no justification for blocking publication because it would interfere with his work.

Moving on to your real question: He has a lot of power over you both because he is senior and because he was your supervisor. Thus you should proceed with caution. Perhaps you can reason with him and seek a compromise. If that doesn't work, you can escalate -- appeal to the other editors of the journal, who presumably would not agree with this behaviour.

In the long term, you should consider slowly moving your research away from his to avoid repeating this situation. And you should work with other people. The situation would look a bit different if you had senior coauthors to fight back on your behalf.