Hey Laurie - You could always set up a new tag in Marketo that would accomplish this - instead of "Webinar', create one called "Customer Webinar" or "Customer Training".
Alternatively, you could simply add to your scoring rules that scores webinar attendees, and have it simply exclude anyone who is marked as a customer from being scored.
Hope this helps,
You could also exclude anyone that was a member of that specific program from being scored in your webinar scoring model. I'm not sure how your scoring is set up, but that is definitely an option too.
We do that for certain forms too for our scoring model...i.e. so folks don't get scored for unsubscribing etc.