I think that this is one of the main issues affecting raids: people switching weak defense teams to voluntarily reduce their trophies.
Why people are doing this? Just for 2 reasons: the main one is that in this way they can easily farm insane amount of food. The second reason is to get an easier 40 Heroes chest.
To avoid this behavior I think the only way to do it is to create an additional ladder, a weekly/monthly average trophies ladder.
Rewards would be only food and iron.
In this way, if you want to dump your trophies you are free to do it, but people staying always high in ladder will be anyway incentived to stay where they are because there will be a loot of food/iron coming every end of the week.
Hope this is clear.