Why is the annoying "sign up for Thai visa newsletter" popping up even when cookies have been enabled before this particular session and I've already signed up for the newsletter. Surely there is a script that will check the cookie whether this annoying nuisance has been presented to the person before and even be able to correlate the username with their email address and check the database. Surely. Enough already!

It's probably an off-the-shelf script, like a lot of other scripts that ThaiVisa uses. Doesn't have the capability built in to do what you're asking.

Also, newsletter membership is stored on the server side, nothing about it needs to be in the website member cookie. So wouldn't work that way anyway.