This can't be disabled. When the phone is charging via the magnetic charging dock and you answer a call, the loudspeaker will be enabled. But once you remove the phone from it, the sound will automatically go via the ear speaker. It's not meant that you should lift up the whole charging station for answering a call.
I had the same question. It is a pity that it's impossible to disable auto loudspeaker mode. I've ordered magnetic calbles, to reduce opening the side panel and save waterproofness for long time.
