You know that you are not blocked, you are not posting anonymously, and you still cannot tag them? That could be a glitch. If you want, you can let me know the post where this happened and we can look into it :)
That's happened to me before too. I've copied and pasted the username to make sure I got it right but for some reason it just doesn't tag them and I know I'm not blocked because they've usually just commented on my post :P As far as I'm aware, you can't turn off the tagging feature, it just might be another one of GAG's glitches.