KMail 5.7.2 Plasma 5.12.2 Frameworks 5.43.0 Qt 5.10.0 openSUSE Tumbleweed 20180305 Kernel 4.15.7-1-default When adding new email accounts to KMail, it asks whether it should automatically create an OpenPGP-key. I added three accounts so far and deselected this option every time, but it still created keys.
So it seems this was a mistake by me, because the combo box also needs to have "no key" selected in setup obviously.