How to (temporarily) prevent yum / dnf from updating repositories?

I know this question is a little old (and this option might be new) but this just worked for me:

 -C, --cacheonly
              Run entirely from system cache, don't update the cache and use it even in case it is expired.

So dnf -C install ... worked for me

p.s. in my case I was installing RPMs that I downloaded elsewhere and transferred over sneakernet because my networking is busted and I did not want to subvert dnf by installing directly with the rpm executable

If you look in the various dnf and yum repo config files you should find several explicit metadata expiry times, eg:

/etc/yum.repos.d/fedora-updates.repo
  metadata_expire=6h

/etc/dnf/dnf.conf
  metadata_expire=86400

You can override these on the dnf command line using --setopt=, but you must explicitly do it for every enabled repository, as well as the dnf main configuration. So you end up with something like

sudo dnf --setopt=metadata_expire=-1 \
 --setopt=fedora.metadata_expire=-1 \
 --setopt=fedora-update.metadata_expire=-1 \
 --setopt=rpmfusion-free.metadata_expire=-1 \
 search abcdef

Note the use of sudo to avoid dnf creating a separate cache for the user.