If you are busy at work, then the easiest way is to not respond until work day is over. I don’t think it’s necessary to even tell her not to bother you. Just don’t respond.

As about asking about fiancée, I don’t see any reason to make a big deal about it. She might be just making small talk conversations. Or she really isn’t that smart. Ton of people have below average intelligence (I try to avoid calling them dumb but I think that’s what you mean), there is nothing that you need to do about it. It’s just what it is