If people don't have a flag to do it with they'll just pick something else, even if it's arbitrary. The issue with harassment isn't one that stems from the flags, that just gives them a specific topic instead of an arbitrary one. If you had no flag, the same people would just call you a no-lifer or ugly irl or something like that. Blocking after 1 annoying message is the solution that fixes all of those pms, not just the flag ones.