Fully agree with Peter, very easy to bypass IP blocking these days, there are some sophisticated systems that can still detect but mostly outside the range of us mere mortals!
If you block a particular country from crawling your website it is pretty certain you will not rank in that country (which I guess isn't a problem anyway) but I suspect this would only have a very limited (if any) impact on your rankings in other countries.
We have had a similar issue, here are a couple of ideas.
1. When someone places an order use a secondary method of validation.
2. With the new customer entries/registrations make sure you have a good captcha, most of this sort of thing tends to be from bots. A captcha Will often fix that problem.