Assuming that you are using ver 1. and are using a standard dashboard,

As an administrator, enter Sharepoint via the Dashboard interface.
Select the tab called Management. On this page there is a section that
lets you manage subscriptions  for any user in Sharepoint.

Remember, SharePoint (v 1) authenticates against the AD, but maintains
it's own users list and permissions.

After you remove his subscriptions, go to the Management console and
remove him as a user. Actually, maybe just removing him as a user will
solve the problem, I don't know.

This is not possible, because you cannot manage subscriptions  on an account  that
is not present

I've been able to manage any subscriptions  for users even though they've
been removed from the domain. In the admin dashboard there's a webpart
for managing  subscriptions. Just enter the domain/userid to access it.
SharePoint doesn't seem to validate the information against AD, it just
looks up the subscription  in the webstore and lets you delete the
subscription in the webpart.