I thought I would turn off automatic cross-posting and use the cross-posting options available for pieces already posted in the web interface instead. Unfortunately, the accounts I would crosspost to disappear from that interface, unless I have automatic cross-posting turned on. 
I just tried to use the web post feature from where one can select crossposting options inside the composition window, because I had forgotten about that possibility. The cross-posting checkboxes there only appear if I have automatic crossposting turned on. When I disable such automatic crossposting, I no longer have those crosspost options in that composition window. It seems the choice is only a choice if the default is to have crossposting turned on. Have I understood correctly? @help
Never mind, I finally see it now. I was trying to disable automatic crossposting by disabling specific accounts. I only now saw the disable button above all the accounts. Maybe a note on the page about what the various buttons there do would help, perhaps with such notes attached to little question marks we can click or mouse over. Or maybe the problem for me was the use of the word “disable” in both places. @jsonbecker tried to tell me this feature worked yesterday, but it didn’t work for me because I was disabling the wrong thing(s).