Removing a Team Member from your Group is a pretty simple process.
First, it is important to make sure that you reassign any Phone Number associated with this member to another Call Flow or user, if you want to keep the number. If you don't reassign the number, any call to it will stop working. To learn how to reassign the number, check out this article. If you don't want to keep the number, just ignore this step.
Now, head over to your 'My Team' page located under 'My Settings' on your left menu.
Look for the Team Member that you want to remove, and click on the dropdown menu under the 'Details' column. You will find the 'Delete Account' option there.
Once the member is removed, any phone numbers assigned to him will get deleted and you won't get charged anymore for this user account.