Copy this script into ~/bin/:

#!/bin/bash
# Removes old revisions of snaps
# CLOSE ALL SNAPS BEFORE RUNNING THIS
set -ex

LANG=en_US.UTF-8 snap list --all | awk '/disabled/{print $1, $3}' |
    while read snapname revision; do
        sudo snap remove "$snapname" --revision="$revision"
    done

Found on https://www.linuxuprising.com/2019/04/how-to-remove-old-snap-versions-to-free.html and slightly edited.