To remove a member's permissions:

• Go to my groups > manage > manage membership > member permissions
• Locate the member's name under the permission you want to remove
• Click "remove..."
• Click "Yes" to confirm your choice

If you would like to suspend someone's membership completely (to prevent him or her from accessing your group at all), you can find out how in our related article on the subject: How do I suspend or remove a member from my group?