There are 2 options, but you might have to go to your router settings.
Have a moderate or strict NAT instead of an open one, if i remember correctly you cannot host with these, but it can affect matchmaking or connecting to parties (especially on strict). It's not a smart thing to do because some other MP games might not work properly. If you want this, disable UPnP, port fowards or DMZ either on the router settings or on your PS3's network settings depending on what you use to open ports.
Other (better) option is to use QoS on your router to limit the upload bandwith that will be available for your PS3. You might have to go as low as 300-500 Kbps in my own experience to avoid host consistently (i have a really stable and decent high speed cable connection and i get chosen as host all the time, and i get horrible lag while hosting in BO, MW3 and BO2 so i've tested this quite a few times to avoid host). And obviously use MAC or IP filtering to apply this only to your PS3 instead of your whole network (i'm not an expert on these things but at least on the DD-WRT firmware i use there are options to apply it to all traffic going through the router).
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