What happens if a merit source stops meriting posts? Are they booted out?
Only if that merit source sends a request to be blacklist by theymos (blacklist ~ no longer want to be a merit source) or if theymos reshuffles merit sources that he barely does.
Because i feel as a merit source it is a duty to you to merit posts, if you are a member that is not a merit source then that is different, you might not give out any merits and it would not be a problem, only that members may not merit you too as you are not involved in the system, but as a merit source deciding to stop meriting posts sounds like 'quiting your job'.
Personally, it is not a duty or a job!
Merit sources are doing free meriting jobs but they have to spend their time to do it. So there is no pressure from theymos I think. A merit source is granted if theymos considers that member is a quality member and has ability to clarify bad and good posts, trusted enough to not sell sourced merits to get money etc.