If you have your facts right, have documented proof and a great lawyer then anything is possible.
There have been many collection agencies successfully sued over the years by debtors. Damages have been awarded and bill collectors have been fired. It doesnt happen very often but when it does, the message is heard loud and clear by the collection industry.
Here is one:
http://canlii.com/eliisa/highlight.do?text=Paulin+v.+P.C.M.+Collections+Ltd&language=en&searchTitle=Search+all+CanLII+Databases&path=/en/on/onsc/doc/2007/2007canlii51169/2007canlii51169.html
If that link doesnt work go to canlii.com and search "Paulin v. P.C.M. Collections Ltd"
Looks like Paulin won a $475,676.52 judgment on this one
Enjoy.