There is a function for this type of thing called
force_unmodified(). I use it on certain maps to prevent removing critical sprites that would give the cheater a significant advantage.
You do not have access to a client's file so you wouldn't even be able to get an MD5 or any other hash of their files. I'm not sure how the aforementioned function works but it's probably something built into the game.
I use the "force_exactfile" type and simply put in arrays of zeros for the second and third arguments ("{0,0,0}"). I have the function called in plugin_precache() but I can't remember if that is a requirement (I suspect that it is).
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