For completeness sake: I have just created a separate VLAN for the port on which the IPTV is connected and added that VLAN to my bridge interface. IPTV still works as it should, and no more IPTV multicast traffic arriving on my PC or server. Since the IPTV device doesn't really communicate with other devices within my LAN network, the downside of slower forwarding performance shouldn't matter at all. 
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