Only the team admin can remove another member from the team. From the People tab on your Sookasa Dashboard, click the gear icon that appears next to the member's name, and choose "Remove user" to remove this member from your team. The remove user will not longer be able to access the team's encrypted files.
If you also want to remove them from Dropbox folder sharing, you can find instructions here. Keep in mind that even if they're still part of shared folders on Dropbox, they won't be able to access any encrypted files because you've taken away the keys on Sookasa.