I know daycarebear tried a 'parent registry' where providers could do 'beware of care' type scenarios - but not sure how it works cause have never used it and well you have to click you obtained permission to search their phone number and well i imagine clients like THIS who hop daycares are not going to GRANT that

http://www.daycarebear.ca/registry.php

If your area has a local home childcare network that meets to discuss business and improve their programs and so forth might help to join up with something like that - cause sometimes 'word of mouth' about nasty clients can make its rounds around those so you can at least make an 'informed' decision about taking someone with a bad rap on.