First of all, I think it might be easier to define what constitutes “correct input” and remove everything else. For example:
input = input.gsub(/[^0-9A-Za-z]/, '')
If that’s not what you want (you want to support non-latin alphabets, etc.), then I think you should make a list of the glyphs you want to remove (like ™ or ☻), and remove them one-by-one, since it’s hard to distinguish between a Chinese, Arabic, etc. character and a pictograph programmatically.
Finally, you might want to normalize your input by converting to or from HTML escape sequences.