I would be tempted to turn that on its head. Use the annoying maths as a soft punishment. If the players want to pick up everything, tell them they can – but first they must work out the encumbrance and show the results to the DM. While they are doing that the DM can have a nice break or prepare the much more interesting next section (perhaps occasionally muttering things like “cool!” to herself).

Once they get back to a merchant make them work out the value. Also tell them the merchant doesn’t want all of it.

If this works then the players will start to avoid mass looting because it is boring for them and their time is better spent elsewhere – having adventures.

Are they properly fencing the loot? If they hang on to it, they run the risk of “locate object” spells tracking them down. If they try to sell it on the street, they’ll be arrested for possession of stolen goods. So just what does happen to all this stuff they’re taking?

You can also try to enforce encumbrance rules, but that’s a lot of effort on the DM’s part. Your best bet, honestly, is to explain the situation to the players, and then play true to the setting.
Brian´s last blog ..Honey Cakes for Cerberus

In our case, we have a mostly city-based fantasy RPG adventure where there are… guards, innocents, not-so-innocents, and the party… and if honestly if we hung around too much longer in the last session after a particular battle we probably would have been arrested by the guards, which might have made an impact on the looting but we ran against a hard time stop for the session.

The post-apocalyptic setting sounds like fun… Sort of a Fallout 3 or Book of Eli approach?