Log in to the site admin
Delete the group "Cart Users" from the Manage Groups
Delete all shopping cart components from the modulestore
- fakegateway.dll
- flatrateshipping.dll
- gateway.dll
- gateway.dll.config
- gcheckout.dll
- shoppingcart.dll
- standardvariant.dll
- weightbasedfulfilmentprovider.dll
- Any other DLL's installed for the shopping cart (gateways, other shipping providers, etc)