Don't know about you other affiliate guys and girls, but I sure don't like the idea of giving easy money to the parasitic leeches (voucher code sites). We do the "content", PPC, and we know a crappy voucher code site is going to take 10% or more of our commissions.
No more. We no longer promote merchants with prominently positioned voucher code boxes. Example - Craghoppers - big box, "Promotional Code, enter it here". Fieldandtrek, orvis, rohan, 3 mobile, mobiles and many many more.
There's likely a correlation between box positioning (and wording) and the percentage of visitors who then go off to Google (brand + promotional code etc).
Voucher sites are parasites. Leeches. Low value spammy Web sites. A big chunk of their traffic comes from the dopey merchant stating "got a code". Gee, no, let me go to Google and have a look.
What percentage leaks out for each merchant? Positioning plays a big part - plastered in the middle of the screen (like Craghoppers - don't mean to pick on you, I love your gear

), or put to one side.
Voucher code mailing lists? Only morons and/or other voucher code site owners looking for some codes sign up to them. The real penny-pinchers sign up to cashback sites.
Solution
If a voucher code is not valid - do not reward the voucher code site. This should mean at least 90% reduction in their profits. Invalid codes are ones which have expired, made up (lots of made up ones floating around - just claim "user submitted" and make them up all day long) or a code not meant for the online channel.
If the referrer is a voucher site and a valid code is not used - 0% commission, and reward the prior referrer. Better yet, don't credit voucher sites with sales at all. Ban them.
Some people wonder why the "affiliate marketing industry" is held in such low regard. It's because 99% of sites trying to get a commission are utter crap. Low-value, spammy, crapfests - voucher code sites are prime examples. Yes, there are some OK blog-style voucher code sites, but the business model is still of the parasitic leech low-value variety. Very few people are going to visit your little site again unless they find it in a search - and most of the time your codes are invalid, getting the visitor to click when no valid codes exist.
All this "please give us a voucher code because we're great" nonsense - pathetic. Affiliate networks posting voucher codes - "here you are affiliates, put this on your silly little Web sites and get a commission" - pathetic.
It's like a pyramid scheme - a few people at the top smooching with network staff and merchants, asking for "voucher codes", so they can sit on their asses and get easy commission. Pathetic.
That ASOS guy spoke a good deal of truth. Not sure I would have used those words in an interview, but he spoke what a lot of people outside the "industry" think - mostly low value Web sites getting in the way of a sale.
For those with OK or decent sites, more often than not they see their cookies over-written by voucher code sites, brand bidders, cashback sites, the networks themselves creating competition -
AW cashback

and other stuff. Having merchants subtly encourage the visitor to go searching for a code is another winner.
Networks: Must be nice getting all those top commission tier payments from brand-bidders, cashback and voucher code sites. Much better to get 500 sales on Vodafone's top-tier, rather than 500 sales spread out amongst more lower-volume affiliates.
Merchants - stop pussy-footing around with silly little policies - "the voucher code site must not use the words 'click here' when there are no codes". I can reword the big call to action box without mentioning codes at all - 'Check latest promos' etc, and still get an excellent CTR.
Affiliates - stop being chumps and lining the pockets of lazy-ass parasitic voucher code sites. Stop promoting sites with voucher code boxes until a proper solution is found.
Banning voucher code sites is the best option. Several benefits:
1. Get respect for your big balls attitude from all non-voucher code site owners.
2. Save time - no monitoring voucher codes sites to see what other douche-bag tactic they're using.
3. Save time issuing them.
4. Do your bit for the Web - stop encouraging people to build lame Web sites.
OR
First referrer gets the sale - but the networks won't do that. Less profit.