In your case, I think, the user has posted some symbol which consists of the numerical code 194. In Latin encoding, this symbol means Â; if you switch encoding of your forums in your browser from UTF-8 to Western, you will see what I mean: there is such symbol separated by line breaks which makes the post passing through. It's not an empty post, you only think so because you don't see anything typed.
I am not sure if this problem belongs to miniBB and if I should fix it (because in general I don't know how to fix it - and I don't know what else symbols can be involved in repeating it, there may be the symbols with codes 195, 196 etc. as well). On the other hand I don't really care about it, because it's more related to users content, and excuse me, but users can also post something not suitable with your board rules, "bad" words or "bad" content, they can type something stupid like simply "A" letter for message and topic title if it's allowed by your forums settings - you will not try to prevent it using text filters, will you?.. And you can't. It's a moderation question.
So the most effective solution is just to ban this user from further posting, as I mentioned earlier, and delete this post. No other solution is useful.