Streams to ClusterSharding Sink - actorRefWithAck

I want to avoid spamming messages to a shardregion that cannot handle the throughput (or starting to discard messages because buffer size has been reached)
Is there any way I can solve that?
Currently I use an actor in a middle actorRefWithAck, still I cannot use ack/? with a shardregion it just discards the message if it reaches the total buffer

Thanks in advance,
Ricardo Marques