If you would like to set a number of your customers to "Inactive" in bulk, this can be accomplished via the Customer Import tool.
IMPORTANT: Please be mindful of using the Customer Import tool to edit customers in bulk as it it possible to erase or accidentally change large swaths of information inadvertently without recourse. Please read this article for details on proper protocol, including downloading a backup of your customers prior to making changes via import.
Find And Export Your Customers to Deactivate
In order to select the customers you want to deactivate, you might find it easiest to create a Customer Filter to find them. For example, if you have a group called "Dormant" that you've been funneling non-purchasers into, you could filter for this Group to get a list of those customers.
After filtering, select them all using the "Select All" checkbox, and then under the "Choose an Action" menu select "Customer Data Export [CSV]"
Delete all Irrelevant Rows
Delete all other rows INCLUDING THE HEADERS except the following
id
active_account
deactivation_reason
deactivation_note
IMPORTANT: Please make sure that no other column headers are in your CSV to avoid overwriting the data in those fields.
Fill our your CSV
active_account: change it from Yes to No
deactivation_reason: enter a case-sensitive match of an option you have entered in Settings > Configurations
deactivation_note: if you would like to add a further note
Upload your CSV
Save your CSV as an MS-DOS formatted CSV and upload it via the Action menu on the Customers page. You may want to try with a few customers first to ensure the upload is behaving as your expect.