How do I blacklist certain characters in Tesseract

[ckalinwi]

I'm looking to block specific characters that I know will never, ever, ever occur in the subtitles I'm converting. I tried the published way for stock Tesseract (a config file in tessdata/configs) but that doesn't seem to work. What am I missing?
To be specific - I don't (think) I want to do a replace, because each character in question that I want to block might actually be one of two other characters each. (Like "H" could be "B" or "8", as a generic example - note, these aren't the actual characters I'm talking about).
I want Tesseract to be told "it's not possible that you think it's character, try the next character you think it is".
Is that possible?