You don't nessecarily have to avoid them dropping XP, you just need to clean it up afterwards. There's nothing stopping you plonking an XP turtle down set to auto-collect and waiting to see whether RichardG's code screws up if the numbers go too high.
My main questions would be situational. Are you doing this in the nether and hoping to avoid aggro'ing the hoard? Do you have a T5 shard and just letting them fall?
For the 1st, I'm not sure. It's been a while since I've made a spawn-trap, and not sure if I remember any of the mechanics.
For the second, the methods that pop to mind are:
1) Turtles. Wash the piggies to the turtles, chop them up, pump them out and use the aforementioned code-checker to clean up the XP (when I did this the XP was the object, and the masses of diamonds I ended up with was simply a game-breaking side-effect).
2) Golems. Not entirely sure how the little guys would fare against armed foes, but they should be able to hold their own. The upside is that you can have a scurrier or two picking up the items.
3) MFR stuff. Not confident in the mod, but I know there are options.
4) Tesla Coils. Always a popular option. Find a method for cheap power and watch the piggies burn!
5) Quicksand. Not quite as impressive to look at, but the nether-dwellers will suffocate without bothering you too much (assuming that you can handle the noise).
6) Cactus. The old vanilla trap, but be careful with those drops!
7) Dropping. The easiest and most effective method. Let them spawn at great heights and watch them try to fly. For added enjoyment make a 24-block tall tower out of glass viewers and watch the little blighters drop. I'm sure they did something to deserve it.
Personally, I've gone turtles for a rather complicated, but effective single-mob system, or a huge drop tower with a range of spawners to keep my systems stocked with copious quantities of goodies.
. Any ideas?