>I think there used to be a way to put something like this in the
>configuration file for yum, and I thought dnf as well.  But I took a
>quick look at the man page for dnf.conf and didn't see anything like

Indeed you could add, for example, exclude=kernel in yum.conf.
According to the manpage of dnf.conf, using exclude is also valid in dnf.conf; see, for example,

http://www.dsm.fordham.edu/cgi-bin/man-cgi.pl?topic=dnf.conf&ampsect=5

I need to say that I did not try it with dnf, only with yum.

Regards,
Rami Rosen