M4300/M4250 - Keeping multicast on switch

I have several M4300 / M4250 switches set up for a customer with various Crestron NVX implementations, all of which is working well.

We have a very basic connection to the customer's existing network by simply connecting a standard gigabit port on the M4300/4250 (port 48) to one of their switches, used only for remote access and diagnostics. We are not directly trunked to their switches or stacks, in effect we appear as a simple unmanaged switch to their equipment (BPDU filter, disabling STP on that port - I know, I know, not good things to do, and will ultimately be resolved properly).

We want to make sure multicast does not leave our dedicated M4300/4250 switch, and block all multicast traffic from leaving (or entering) port 48. In effect, I want to make sure all multicast is blocked inbound and outbound on that single switch port, but allow other TCP/IP traffic for remote diagnostics, etc.

I've tried using the Configure>Multicast -> Block Multicast method in the AV GUI webpage:

But I still see multicast traffic entering and leaving the switch on port 48:

Does anyone have any suggestion on ensuring that all multicast traffic (even if it's by IP/range) stays on the switch and does not traverse my "uplink" port 48?

Thanks

Jay