My friend's start-up sells RFID-shielding wallets, passport cases and apparel. difrwear.com

Wonder if they can't they be disabled by a strong EM field? That's basically how the RFID tags on merchandise are disabled at the retail checkout counter.

I use aluminized Mylar potato chip or ramen bags for this. I've verified that they work with a FasTrak transponder, but I haven't done measurements to see how much the bag really attenuates the signal.

Are these documents invalid of the RFID chip is destroyed?

I would think so if they were using the chip to authenticate the document.

I think I read somewhere that intentionally destroying the chip is a no-no, but if you 'accidentally' hit it with a blunt object and it stops working the document is still valid. Don't quote me though.

I recall reading something similar with regard to the new, chipped U.S. passports. If the chip fails, the document is still valid. (Though I expect you might well be in for some hassle. And don't take my word WRT the document's validity; I'm just remembering some random thing I read.)

I would imagine, though, that intentionally disabling it represents some form of criminally liable tampering or vandalism. Hopefully not forgery; I seem to recall that forgery carries some big penalties. I wouldn't see disabling some of the information as forgery, but I don't know how a prosecutor might argue the situation.