By Catherine Belton, Paritosh Bansal and Megan Davies LONDON/NEW YORK (Reuters) – A decision by Western allies on Saturday to block “selected” Russian banks from the SWIFT payments system will inflict a crippling economic blow, but also cause much pain to their own companies and banks. And the allies still have room to do more. The Society for Worldwide Interbank Financial Telecommunication (SWIFT) is a secure messaging system to ensure rapid cross-border payments which has become the principal mechanism to finance international trade. Russian banks denied access to SWIFT will find it harder t…