Which IPv4 feature prevents multicast joins on a per-port basis?

MLD (Multicast Listener Discovery) is the IPv6 equivalent of IGMP and is therefore not the correct feature for controlling IPv4 multicast joins.

IGMP filtering is an IPv4 feature that allows a switch to explicitly permit or deny IGMP join messages on a per-port basis, effectively controlling which multicast groups hosts connected to that port can join.

IGMP snooping optimizes multicast forwarding by preventing traffic from being flooded to all ports; it monitors join requests but does not actively prevent or filter those requests on a per-port basis.

PIM (Protocol Independent Multicast) is a multicast routing protocol, and PIM snooping, while it might optimize PIM control plane messages, does not directly prevent host multicast joins on switch ports.