I can't seem to find a config option for discarding all forwards.
There's deliberately no way to refuse to forward payments over "public" channels because that's the definition of a "public" channel - one which you are saying you want to accept forwards over.
If you have opted to open private channels (not announced) then
accept_forwards_to_priv_channels is what you want (and keep the channels private!).
You can read more about that in our API docs. https://docs.rs/lightning/0.0.113/lightning/util/config/struct.UserConfig.html#structfield.accept_forwards_to_priv_channels