People can't be totally deleted from your account as they might have commented or collaborated in projects in some way. So instead of deleting people from your account you can archive them.
When you archive someone their past contributions to your account all remain in place. So discussions, files and any other information all remains. However, they won't be able to login or access your account any longer. And, they won't count towards your paid user seats.
To archive someone, go to the main people page and find the person you want to archive. Then click the options button next to their name and click "Archive Person".
When you do this you'll see a pop up explaining what will happen if you archive the person.
If you are happy with this then you can click on "Archive" to continue to archive the user. Or if you change your mind you can click on "Cancel".
If you archive the person then they won't show in the "Active" people view any longer. Instead they will show in an "Archived" view. To access this go to the main people page and click on the "Active" button and change the view to "Archived".
You'll then see the archived person in this list.
If you want to restore the person then you can click on the "Options" button next to their name and then "Restore Person".
This will restore the person so they can access your account again. They will then also show back in the "Active" people view again.