How to remove uninstalled snaps from cache?

You can remove the files in /var/lib/snapd/cache without issue. Also there is no need to stop snapd before.

This was answered in Snapcraft forum:

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The answers boil down to: you should not have a lot of files with hardlink count 1; at most 5 in the default install. If you have more than that, it’s a bug, please let us know.

And yes you can remove them without issue; no need to stop snapd.

Here the command to do that:

sudo rm /var/lib/snapd/cache/*

You can also simply do:

sudo snap remove --purge $PACKAGE