You got it backwards. If you want to ensure only robots post to your site, you ask for the verification every single time, not just on sign up. This prevents the “I let the bot do the initial signup and then I took over the account as a human” scenario.

But since you built the bot, you can have it do anything? Answer all HATCHAS and then do anything according to my instructions?

Simply use a browser plugin (or any other mechanism) that invokes the agent when the "HATCHA" appears.