"Using the occ command" documents using versions:cleanup either globally or for specific users, with example invocations for both cases. However, said examples are identical in the current revision:
This example deletes all versions for all users:
sudo -u www-data php occ versions:cleanup
Delete all versions
Delete versions for users on backend Database
[…]
You can delete versions for specific users in a space-delimited list:
sudo -u www-data php occ versions:cleanup
Delete versions of freda
Delete versions of molly
Unless I'm misinterpreting this, the latter example should have the space-delimited list of users following the invocation, like so:
sudo -u www-data php occ versions:cleanup freda molly