Need help in blocking spam at Facebook public group

[Happy Hopping]

So for those who run a public group at facebook, the way that facebook works since last year, is that you don't have to be a member to post.

So at group setting, I always have: Participation: Participant Approval, Review All is set to ON

but there are some who still manage to post those video spam w/o being a member

What can be done?

Is there a spam filter that can block any new member or anyone from posting for the 1st time and hold it for review?