I never knew a company had a "birthday"!
Have you tried control-click->Delete on the group.
If 'control Delete' leaves a bunch of individual contacts, I would think just deleting any/all dates in a contact would suffice.
Not sure why you want to blame Apple for the way a "social" app creates a list of calendar entries. It's just another invasion of our apps, data and habits by commercial entities. Guarantees you get a "reminder" of every LinkedIn contact... for ever!
Personally, deleting the entire group sounds like a perfect solution. I think it is rare for a company to not have a web site these days. Create a bookmark in URLMPro. If I want to contact any of them I'll simply visit their website/contact page.