Removing old snaps on Ubuntu

How to remove old snap versions on Ubuntu to free up disk space.

To see what snap volumes are currently mounted:
mount | grep snap or snap list --all (thanks to ErsinErtan!)

Example result:
Name                  Version     Rev       Tracking   Publisher  Notes

core 16-2.32.8 4650 stable canonical core,disabled
core 16-2.33 4830 stable canonical core,disabled
core 16-2.33.1 4917 stable canonical core
pycharm-professional 2018.1.3 66 stable jetbrains disabled,classic
pycharm-professional 2018.1.4 68 stable jetbrains disabled,classic
pycharm-professional 2018.2 78 stable jetbrains classic
rubymine 2018.1.3 25 stable jetbrains disabled,classic
rubymine 2018.1.4 32 stable jetbrains disabled,classic
rubymine 2018.2 39 stable jetbrains classic

In the example above there are several versions of PyCharm. Revisions 68 and 66 are no longer needed.

Uninstalling older revisions:

sudo snap remove pycharm-professional --revision 68
sudo snap remove pycharm-professional --revision 66

Repeat the process with all the other snaps if needed and that's it!