To delete members from the team, open the Team settings > Active Users tab. Open the context menu of a team member and choose Delete from team:
Delete team member
If the user is the owner of some boards/templates created in this team space you will be given a choice whether to accept the ownership or delete them:
Changing board ownership, while removing a user
The chosen user will lose all access to your team projects right away (without being notified). Please note that they will retain viewer/commenter access to the team boards that were shared via a public link - if the user saved the links to these specific boards.
Removing members on the Team, Business plans
As an admin, you can add or remove team members at any point during your subscription period. After the removal of a team member, you will not be immediately refunded for the time remaining in your current subscription period. Instead, there will be a vacant seat in your team. Extra vacant seats need to be removed additionally.
⚠️ Removing team members doesn't decrease the number of paid seats.
You may decrease the size of your team in the Team settings > Billing information > Overview. Click Change team size, set a new team size and click Confirm. You will see a message saying that your account size is now changed and your next purchase will be prorated for the new number of seats. The prorated amount for the unused time will be added to the subscription balance.
Decreasing the number of paid seats and changing the cost of your subscription
To know more about the prorated system of charge, please check our article - Understanding Billing.