There are no "M-" characters as such. That's just how cat is displaying the non-ASCII characters. The ones that you say are "trivial" are probably some kind of white space character that you don't notice in the spreadsheet.

Once you know what characters they are, for example as suggested in Re: What's the 'M-' characters and how to filter/correct them?, you can remove them with a regular expression. For example, here's a situation I dealt with recently involving invisible special characters that were causing problems with web browsers:

        # U+2028 ('Line Separator') and U+2029 ('Paragraph Separator')
+ are valid JSON
        # but cause a parse error in the browser. So we remove them.
        $job_xml =~ s/\x{2028}|\x{2029}//sg;
[download]

update: showed recent example

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