I cannot reproduce this issue at all. I get the Greek preview only if I have a complete Greek glyph set in the file. Just those four glyphs, nothing. Tested in Word 365 on Windows 11.
Perhaps it is a bug that shows in Word on Windows 10. If so, the best solution for your client clearly is to update to Windows 11.
If that is not possible for your client, I would argue that the glyphs are wrongly encoded. Rename them:
Omega→OhmDelta→incrementmu→micro
pi ist the only one I’d keep the same.
Or, if you really want to keep the Greek Unicode values (which is probably not what you want if you want to work around the Windows 10 bug), run the Double Encode mekkablue script.
UPDATE: I was able to suppress the Greek preview word in a font that had all Greek glyphs by disabling the Greek bit in the unicodeRanges parameter. Best practice: reopen a TTF export in Glyphs, copy the unicodeRanges parameter from Font Info > Font, and paste that in the Font Info of your original .glyphs file. Open the pasted parameter there and disable the Greek bit. (That way you can keep all the other bits as they were.)
Perhaps a combination of renaming the glyphs and disabling the Greek bit is the best solution.