From Plant 13 (maybe 12.2 also, not sure), you can also de-activate the Processing-Importer independently from the Setup-Importer. But in early versions there was a bug and this setting could not be saved, it would always go back to both importers active. So make sure that you are using the latest patch!
I am using plantsim 11 and i dont have this problem. I can choose seperate operator for setup and seperate for processing.
Open importer tab:
uncheck common resources--> click setting up radio button-->open service for setting-up-->put service "Setup" & amount 1--> apply-->click processing radio button-->remove all service/delete table content-->apply-->ok
CLICKING APPLY AFTER SETTING UP IS IMPORTANT.
put a broker and worker pool with operator service as "Setup" also in the model. Remember to put broker seperately in both the services. Check the model attached.
I just wanted to let you/the OP know about a new option, in case you/the OP are trying to use it and the bug of the first versions is causing the issue (since the OP didn't state which version he/she uses or what exactly he/she did, I thought that bug could have been the issue, because of the symptom "as soon as I close the single-proc window it comes back to processing").
Here's what I was referring to: you have individual checkboxes for both importers, to (de)activate them individually: