Or, because there are bigger problems to solve, you just let it go; it's a fact that users will ignore anything you write into a messagebox anyway.
Use them sparingly; users can't read and even if they could, they wouldn't want to.
Developers are forever seeking ways to put more and more text and info and notifications into programs because detail and information is what they like; this is the exact opposite of what regular users want. Users hate computers and want the least friction way of getting their tasks done with them as possible. Throwing more blockers in their way and things that drag their focus away from what they're trying to achieve, to perform some trivial task (like disabling the whole program until they click OK on a messagebox), just makes them hate computers even more
(To give an example for this particular context; the windows calculator does not use a messagebox to show you the answer, nor does your phone, nor any pocket calculator you ever used. Any time you want to use a message box your first thought should be "how can I design things so that I don't use a message box?")