Quoting official Phabricator text:
Be careful when deleting users! This will permanently and irreversibly destroy this user account.
If this user interacted with anything, it is generally better to disable them, not delete them. If you delete them, it will no longer be possible to (for example) search for objects they created, and you will lose other information about their history. Disabling them instead will prevent them from logging in, but will not destroy any of their data.
It is generally safe to delete newly created users (and test users and so on), but less safe to delete established users. If possible, disable them instead.
Disabled users are not counted towards your monthly user quota — you won't be charged for an account that is in
disabled mode, as long as it has been
disabled for the whole period.
In order to remove users, go to the Phabricator console in your management panel and use the
remove command, which mirrors Phabricator's
TL;DR It's enough to disable accounts.