Disabling "Members can invite" option prevents also admin members from inviting guests
Setting to OFF option "Members can invite" in Azure Active Directory > Users > User settings > External users > Manage external collaboration settings disables ALSO admin members from inviting guests, not only non-admin members as declared.
So, this an "block all / allow all" option and there is no simple way to prevent standard members (non-admin) to invite guests.
As side effect it produce the infamous "We didn't find a match" message in the search box when you try to add a guest, mentioned by a lot of users.
Please, fix it !!!