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Email addresses are free, so you would really need to build in some kind of clever copy e.g. they put in email address and it says sorry we are sold out and convinces them there are no more copies being sold.
If they really want it they will always find away around it.
Thanks it was a memory limit. Anyone with same issue see Tip 3 here http://www.rackspacecloud.com/blog/2009/12/21/wordpress-hosting-on-rackspace-cloud-sites/
Also to get back into admin just rename the plugin folder.
The following post is also helpful:
After installation of wp-cart-for-digital-products.zip through wordpress interface the wp admin section went blank/white in the latest version of wordpress (3.0.4).
The hosting provider I use is Rackspace Cloud Sites.