|QoS is traffic priortization This is not the same thing as limiting traffic.|
Here where I work we use QoS and give the highest priority (gold) to videoconferencing. The second highest priority (silver) goes to VoIP phones. The lowest priority (bronze) goes to all other traffic. This is done based on the VLAN's associated with each type of traffic. I'm pretty sure QoS won't work for your scenario. You're going to need bandwidth limiting.
I'm not familiar with mikrotik equipment so you'll need to read the manual on whether or not it's capable of limitiing traffic. If it is, then set a low limit on P2P and this will reduce the amount of bandwidth it can use. Without knowing more about the equipment in question I can't tell you how to use it. I know in your shoes, I would read the manual and play with it until I had it figured out and setup the way I wanted it.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.