The cost can (should) be progressive for repeated offenses, and it can be divided if multiple people make the same (wrong) call. Say you get one "free" report per month, but you don't lose it if either you were correct and/or more than 5 people reported the same person.

The important thing is not about being "perfectly provably fair", it is just to curb abuse (on either side) so that people still can have fun playing without having to accept such invasive software on their machines.