As other have said, this is not possible, but because I wanted the same thing without and also did not like returning to a camp with 10 inv spaces empty (which to me translates as lost income) I have been using JunkDelete which will facilitate that. Basicaly you keep on looting everything as usual. When the inv starts to fill up, JunkDelete will hold one inv space open. When next you loot 3 items from a corpse (say, a grey junk, a green sword and a purple armor) JunkDelete will loot each item and destroy a junk item of either lower quality or monetary value. Both of those setting can be edited in game, so you may tell it to delete only greys or both greys and whites, as well as tell it to delete stuff that are worth less than 10s or 10g, whatever you like (those prices being Vendor prices). It works remarkably well, even today.