Dropping Organization Members

There are a few ways to remove (drop) members from an Organization:

Individually...one at a time
Drop all as a batch (NOTE: If you are going to make an Organization 'inactive', you should drop everyone first. Read more about Active and Inactive Organizations.)
From a tag

Individually:

Click on a person's member type within the Organization
Click the Drop button in the Member Dialog box
Answer Yes to the pop up question

Drop All:

On the Organization page, click the Update Members link
Click the Select All box
In the New Values: Membertype drop down select Drop
Click Update Selected

Drop from a Tag:

See wiki article regarding Adding/Dropping Using a Tag