To remove a member from a Team, simply right click on the corresponding user on the planner board and select "Remove User": 


Removing the user from a given Team will NOT impact the other Teams this person may be assigned to. 

You do not need to delete the bookings of this given user before removing him/her from the Team. These bookings will simply be deleted when the user is removed from the Team.

Remember that you may at any time deactivate a user. In that case, the user will be removed from all existing Teams s/he belongs to.