If you need to remove a users access to Wiplist, you can make the account inactive in Settings under Manage Users. Click on the user profile, in the pop up box click the down arrow in the top right-hand corner, and select Deactivate User.
Once a user is made inactive, they will no longer be able to login and you won't be able to add the user to new projects. Inactive users can easily be re-activated by changing their status in the user profile window.
Your subscription includes active accounts only, so you wont be billed for any inactive users. Partial use of active/inactive accounts are pro-rated on your subscription so you only pay for what you use.