Deactivate a client you do not expect to visit again. Deactivating a client blocks Reminders and other automated mail for the client. Deactivating a client also excludes that client from most reports, or gives you an option to exclude.
Do not deactivate a client simply because they have not come in for a while, or have only visited once. Filters on reports and marketing should remove that sort of client from any Reminders list or Report.
Instead, deactivate a client instead of deleting that client. The only reason you would ever want to delete a client is if the client should not have been entered in the first place.
In some cases, deleting client information may result in you losing information you are required to keep under Tax, Privacy and/or Restricted drug laws.