Just add an extra field:
[textarea* your-message2 maxlength:1 default:"h"]
Then, hide this field by adding the following custom css to the Extra CSS tab in your theme:
textarea[name="your-message2"] { display: none !important; }
That’s it.
Now, the bots want to fill out field your-message2 with a lot of info, while it should be exactly the character “h”.
Hopefully, it works.