Hiya, there is no API I know of that will remove a user from all groups, but you can certainly chain APIs. The way I'd do it would be to retrieve the user using the User API, iterate all user.memberships and get the groupId from each one. Then I'd use the Group API to remove the user from each group, using DELETE against /api/group/member?groupId={groupId}&userId={userId}
Hope that helps, thanks for using FusionAuth!