How to remove a contact from a group with Webform?
On the CiviCRM tab of webform, add a contact using Existing contact
Set Enable Groups and Tags to Yes
In the Groups box, choose -User Select-


If you don't want all the groups showing:


Go to the 'Webform' tab
Click 'Edit' for the Groups component
Select 'Static Options' and choose which groups to show


When the form is loaded with an existing contact, the checkboxes corresponding to the existing groups should be ticked.  Untick and save to remove from the group.