There's a few things you can do.
The first, and the one I use, is to remove all non-armor clothing from the game. It doesn't come with the enemy, you can't sell it. To do this, go into entity_default and remove all of the HELM, SHOES, PANTS, GLOVES, and ARMOR tags except for the ones involving leggings, greaves, platemail, chainmail, leather armor, high boots and low boots (just BOOTS and BOOTS_LOW), helmets, caps, and gauntlets. Then, go into the various item files and delete the now-irrelevant entries.
Another thing you might try is reducing clothing's value modifier. In the various item_x files, there are lines reading [VALUE:y]. Cut the y number by a bunch and all clothes of that type will be drastically reduced. For instance, reduce the [VALUE:15] of socks to 5, and suddenly all of those GCS silk socks the goblins bring will be worth only 1/3 as much. Set value to 0 for all clothing, and they'll be totally worthless (though IRC the traders will still give you 1g for them).
Or, for probably the greatest challenge and the most extreme measure, reduce the actual value modifier of everything. Cut the [MODVALUE:z] of all creatures by half or more, reduce the [VALUE] line in all metals and stones by a significant amount, and just in general cheapen everything. Then, everything will be cheaper. The problem with this is that it makes caravan stuff cheaper, too, but since your fortress wealth's growth will be severely retarded you have other things to worry about
All in all, though, I'd suggest just removing clothing from the game and just pretending that everyday wear is abstracted out. Relatively quick, easy, and not too damaging.