This account was premium, premium lapsed. Now the horses that were assigned to the BM are still saying they are assigned to the BM but I have no way of removing them.

This is not a bug.

BM would sign them up again when you´d renew your upgrade
So in most cases this makes more sense than re-assigning them all only because of a possible 2 days break ;)

I don't plan on renewing the upgrade on this account any time soon though? So it's kind of annoying to see the M

So is this just something that I will have to deal with?