They had a quite efficient system to counter this in World of Warcraft.
If your pet(s) was/were the only ones to do damage to the mobs, you didn't get any loot. You needed to damage the mob too yourself.
Lately, they simply also removed aggressive mode from most pets, but the first system described above would work just fine in LOTRO too.
This makes "pet farming" impossible without using third party software, which is of course a bannable offense.