I find your argument to be quite disingenuous considering that the intention of the ostracism it to harm by denying job opportunities and by exclusion from social relations.

Have some decency to call your actions what they are.

If you want to harm somebody in retaliation for doing something you don't approve of, don't call it 'the opposite of harassment'.

I can see your point but perhaps it's a difference of viewpoint. When I am ostracising someone, it's because something bad they did. I think I have the right to choose with who I associate, and nobody has the right to force themselves upon me.

I don't understand why you are redefining a call for ostracism as "deciding who your friends are" and then classify ostracism as "the opposite of harassment".

This pretty much looks like harassment to me:

"the developer community need to start ostracising people working for these companies. Don't hire former employees, don't hang hang out with people who work for these companies and conferences. Don't supply services to these companies (build their website, network...)."

I don't understand what you don't understand. I can't harass someone I don't interact with.