The best approach to credit cards, imho, is the Heroku one: you get a little bit of stuff no questions asked, but you get twice the free stuff if you validate a CC. This way, the user has an incentive to put a CC down even if she’s not going to pay right away, which is good in itself (because you can then leverage it by making it extra-easy to impulse-buy later, as well as improving the quality of your db).
For the record, I am one of those people using + as a tag, just so my username a bit different site by site but still easy enough to remember (yeah I use a password manager, but a little bit extra hygiene never hurt anyone). I’d be incredibly pissed off if sites started trying to be clever with it; I guess I could tolerate a duplicate detector, as long as it doesn’t stop me from signing up as soon as a + is detected.
If you see people abusing it, I would have a “hunting” routine and gently ask the worst offenders to shape up lest they get banned.
The issue with the credit card verification before trying the product is that as a user, I do not like that approach.
I want to try first and eventually pay. That is the experience I'd like to offer.