You can suspend a user at any moment, and that prevents Hubgets from counting it as an account that is invoiced.
A suspended user does not show in the roster, but all content generated by that user in Hubgets will remain available for search and will be traceable.
Given the fact users generate content, it is not a good idea to completely delete a user - as content generated by the user would also be deleted, affecting Topics, Direct Messages, telephony reports etc. Here is how to suspend a user, if you are the Admin/Owner of the organization:
Go to your profile Settings -> Team -> search user and click Suspend/Activate.
I have inserted a print screen below, covering the email addresses of the contacts displayed.