oulook signature file hijacked

I am using Spamfighter in Outlook. It was supposedly freeware, but is actually "trailware". After using the "personal" edition for 30 days, since I did not buy their pro version they added a signature file to Outlook that states that I am using spamfighter but did not buy it, that it has blocked xxx spam emails, and then includes a link their website. They also disabled the signature file option in Outlook tools so I cannot change to a different file. I have tried all of the adware, virus and spyware utilities and nothing has found this hack. How do I fix this? Thanks for your help.

 

A quote from the MG download page for "spamfighter"