Good point, bermeir, about what I would do myself. Actually, no, if someone didn't reply to my letter in some way (letter, email, phone, text), I would assume it wasn't convenient and not turn up. But I don't think it's actually wrong for someone to turn up in the circumstances you describe. If your MiL is the sort of person who isn't very sensitive to what you want, then take her at her word and call her to say it's inconvenient when it is. It really is no use getting annoyed about er way of doing things. She isn't going to change, but you can make your behaviour effective. If you don't want her to visit, tell her that (politely as in "it's inconvenient"). Just be straightforward and cope with what you regard as her unreasonableness. Far less stressful than agonising about it and getting corss.
But from what you say, it sounds as if she just wanted to drop in on her way to or from something else. In the past, people would do that without any prior communication if it was family.
I've had a similar situation with one of my sisters-in-law. I lived a long way away from my brother and didn't see him very often. Once, when I was visiting my mother for a week, I rang SiL to ask if, since we were nearer, I could come over one afternoon so the cousins could play and I could see her and my brother. Her reply was that it wasn't convenient because she had to go shopping. That's code for "No, I don't want you to visit, even just for a cup of tea." I know that because it was my last attempt – there had been a few. I suspect that my brother never even knew I'd called.
It has become clear since then that it's not a personal thing with my SiL. She doesn't let anyone visit. She's the one with the problem. Perhaps if you view your MiL as having a problem (such as wanting to be in control, as you suggest), you will cope better. Good luck with your assertiveness.