This situation sounds like what restraining orders are designed to help with. IANAL, but restraining orders generally cover all types of communication, including social media. Showing that he has continued to harass you even after blocking him is good reason.

“You should stop posting these online, it’s really embarrassing because your [tool name] is such a failure. I should redesign all of it for you because I’m actually an engineer at [company name] and have a lot more experience. I could actually do it right, unlike you. I just might help you if you ask me nicely.”

Out if curiosity, have you ever tried calling his bluff? Reply with how would he improve it by saying any constructive criticism is appreciated. Something like

Do you have any recommendations on how to improve it. Looking to make as good as I can.

Depending on how you’re feeling, you could incorporate tagging their company. Use the same language they used. Something like

I’d love any constructive criticism from an actual engineer from @company

That said, it seems like the time to reply like that has past and it’s best to ignore/block him at this point.