Yeah its annoying as f***. I and another player prestiged a gym to level 8 which is actually really hard to do with the new update only to see a couple of morons waste 2 spots with 500 cp trash.
Its already enticing enough to knock gyms down but when you put trash it just makes it even more so.
I am not gonna bother prestiging gyms anymore until they fix the current leveling up/down gym mechanics.