It's impossible to remove a Service Account from a group in the old user management interface; it generates the following error:

"Unable to remove member - You have limited permissions because you hold the role."

Inspecting the page, the error is:

"Error 400: Org is not vortex enabled"

When accessing a site, it's possible to remove the Service Account from the group, but for Orgs with multiple websites and multiple groups to remove, it would be an enormous task!