I agree with the idea above - put a html "site suspended due to overdue account" type message, and remove the index & wp-config files if it's wordpress so the site pages can't load. Then tell them you'll put it back and hand it over when they pay what they owe, and point out that the longer they take the more their rankings will drop, and you own the SEO that's got them the rankings in any case - since they haven't paid for it - and you'll be turning all that off in 3 days time if they haven't paid. Tell the other web company to go fuck themselves, it has jack shit to do with them. They'll pay up pronto - I've done this with a customer of mine, I had some moron web guy telling me I was being unethical not releasing the site to him - what as unethical as her not paying me for all of the work I've put in, I said - then pointed out he'd be facing the same problems himself if that's how they behave. She soon coughed up.