You can not delete a Member from your Organization. You can only deactivate a Member. Deactivating a Member means they will no longer have access to your Organization.
To deactivate a Member from your Organization, follow these steps:
Select the Organization Administration icon from the top of the page. Select Memberships from the panel on the left hand side of the page.
Under Actions, select the Remove Member icon associated to the member you would like to deactivate from your Organization.
The member is now deactivated.