Note: Only a team's administrator (the user who created the team) can remove team members.

From your team's page, select the Members tab

Find the row pertaining to the user that you wish to remove

On the right side of the table row for the user, click the Remove button

This will immediately remove the user and their sites from the team without confirmation. Tread carefully.

Removing the user will not remove their access to sites previously shared to them through the team. See If I remove a user from my team, can they still access the site?