I don't know how FB currently charges, but if it's a flat rate per click, I'd think a reasonable solution would be make it proportional instead: this user spent

I don't know how FB currently charges, but if it's a flat rate per click, I'd think a reasonable solution would be make it proportional instead: this user spent 100% of their clicks during time period X on you, whereas that one spent only %10. Then you would price based on the share of the user's attention you got, not on individual clicks. Seems a lot simpler than retroactively invalidating clicks from users who pass some arbitrary threshold.